Illustrated Glossary of Colour: A-C

Abney phenomenon/effect

- “change in hue produced by decreasing the purity of a colour stimulus while keeping its dominant wavelength constant” (CIE e-ILV 17-22-070), for example by taking light of a single wavelength and progressively replacing that light with achromatic (white) light of the same visible energy. Because of the Abney and Bezold-Brucke phenomena, lights can vary somewhat in hue despite having the same dominant wavelength and thus the same additive complementary wavelength.

Achromatic

1. Of a perceived colour, achromatic means "devoid of hue" (CIE e-ILV 17-22-049). The achromatic colour terms white, grey and black apply to diffusely-reflecting objects and most transparent and metallic objects, although achromatic transparent and metallic objects of high lightness are more commonly described as colourless. Light perceived to be achromatic is usually described as white light. See also neutral.

2. In the psychophysical sense, an achromatic stimulus is a "stimulus that, under the prevailing conditions of adaptation, gives rise to an achromatic colour" (CIE e-ILV 17-23-009).

Adaptation

Adaptation refers to a "process by which the state of the visual system is modified by previous and present exposure to stimuli that can have various luminance values, spectral distributions and angular subtenses" as well as "specific spatial frequencies, orientations, sizes, etc. (CIE e-ILV 17-22-12), or more simply, any process by which perception is influenced by previous and present exposure to stimuli.

Light adaptation and dark adaptation refer to states of adaptation to moderate (and greater) and very low illumination levels respectively (CIE e-ILV 17-22-15), and can be collectively included under brightness adaptation. Chromatic adaptation is the "visual process whereby approximate compensation is made for changes in the colours of stimuli, especially in the case of changes in illuminants" (CIE e-ILV 17-22-13). Together, brightness and chromatic adaptation normalize to a degree the overall appearance of the visual field relative to the unadapted eye, assisting the (separate) visual task of parsing the visual field into object colours and illumination.

Additive mixture/mixing

- “stimulation that combines on the retina the actions of various colour stimuli in such a manner that they cannot be perceived individually” (CIE e-ILV 17-23-030). Under the CIE definition, additive mixture includes what might be called simple additive mixture of superimposed light beams or of light from the RGB subpixels on a screen (where the stimuli are in effect superimposed from the point of view of the observer), as well as additive-averaging mixture, for example as seen in spinning discs, in which the component stimuli are seen as being averaged over an area. Additive mixing is often loosely called additive "colour mixing", reinforcing the naive assumption that the colours reside and mix in the lights themselves.

Additive primaries

1. In common speech, the additive primaries/ primary colours are the colours of the three stimuli typically used in additive technology (computer, mobile phone and television screens), generally referred to as "red", "green" and "blue" or RGB. The precise colours vary somewhat between different devices but are typically an orangeish red, a yellowish green and a deep or violet blue. Contrary to popular belief, the wavelengths of the additive primaries do not coincide with the peak responses of the human cones (especially for the red primary), but each primary causes one of the cone types to respond more than the other two.

2. In colorimetry, any lights used as components of mixtures are referred to as primaries.

Additive-averaging mixture

- “a form of color-stimulus synthesis in which the result is an average of the components rather than the sum” as in simple additive mixing (Burnham et al., 1963, as averaging color-stimulus synthesis). Examples include stimuli that are moving too fast such as spinning discs (temporal averaging) or too finely interspersed to be distinguished (spatial averaging). See DOC Part 4.4.

Advancing colours

- in painting, the perception of coming forward in the picture, widely said to characterize "warm" colours in the yellow to red hue range. Conversely, "cool" colours are said to recede or retreat. The effect can sometimes be explained by chromostereopsis (q.v.), by which long wavelength sources appear to be located distinctly closer to the observer than short wavelength sources. Another factor that has been invoked is that light, high-chroma colours, which are predominantly "warm" in hue, tend to "come forward" in the sense of attracting attention.

Aerial (or atmospheric) perspective

- progressive change in the appearance of objects with increasing distance. The predominant colour change results from the addition of light scattered from sunlight passing through the intervening air, comprising a combination of bluish Rayleigh scattering from very fine particles (including oxygen, nitrogen and other molecules) and normally whitish Mie scattering from larger particles from the size of a wavelength of light upwards. All natural vegetation emits a "natural smog" composed of aromatic organic volatile substances of the terpene family which are the oxidized in the air to form tiny droplets of tars and resins that contribute to the bluish Rayleigh scattering. The effect is to make the colours of most objects more bluish and/or whitish with increasing distance due to the addition of scattered light, but very bright objects may appear more yellowish to reddish due to Rayleigh scattering out of the light ray reaching the eye. In addition to the colour changes, objects tend to become less clearly defined with increasing distance.

Afterimage

- visual perception persisting after cessation of the stimulus, including positive and negative afterimages. See also complementaries, afterimage.

Aperture mode (of colour appearance)

When a colour is "seen through an aperture that prevents the said colour from being associated with a specific object or source" it is said to be seen in aperture mode of colour appearance (CIE e-ILV 17-22-021) and the colour is called an aperture colour (CIE e-ILV 17-22-044).

Assimilation

- convergence of perceived colour observed when small areas of colour are interspersed. See Part 3.5. Also known as the Bezold effect or the von Bezold spreading effect.

Attribute (colour)

Colour attributes are qualities or characteristics that can be used to classify colour perceptions, for example hue, lightness and chroma. Three attributes suffice to classify colours in a single mode of colour appearance, which can therefore be represented in colour spaces whose three dimensions correspond to the three attributes.

The CIE currently defines six attributes of perceived colour: hue, brightness, lightness, colourfulness, saturation and chroma. Other perceived colour attributes include chromaticness (relative chroma), blackness and whiteness as defined in the NCS, and vividness, depth and clarity as defined by Berns (2004, 2019).

Basic colour names/ basic colour terms

Berlin and Kay (1969) proposed that there are a limited number of universal basic color terms that individual cultures introduce in a relatively fixed order. English is said to contain eleven basic color terms or names: 'black', 'white', 'red', 'green', 'yellow', 'blue', 'brown', 'orange', 'pink', 'purple', and 'grey' (CIE e-ILV 17-22-057, as "basic colour terms").

The category of colours identified by a basic colour name or term is called a basic colour category (BCC).

Bezold-Brücke phenomenon

- “change of hue produced by changing the luminance (within the range of photopic vision) of a colour stimulus while keeping its chromaticity constant” (CIE e-ILV 17-22-071). For example, single wavelengths of the spectrum shift in hue towards either blue (below 500 nm) or yellow (above 500 nm) as they become brighter. Not to be confused with the Bezold effect or von Bezold spreading effect, which are alternative names for assimilation.

Binder

- the substance in a paint, such as oil or acrylic, that holds the pigment together and affixes it to the substrate.

Black

The colour at the dark extreme of the range of achromatic colours, black is the way in which we normally perceive a very low and even spectral reflectance in an object, or a very low flux of light relative to other areas of the visual field. In the ISCC-NBS method of designating colors, black includes colours up to 2.5 in Munsell value (corresponding to a luminous reflectance of 4.6%) and up to 0.5 in Munsell chroma, while colours up to value 2 and between 0.5 and 1.0 in Munsell chroma are designated as black qualified by their hue, for example "purplish black". Pigments that form oil and acrylic paints in this range of values include lamp black (value 0.4 to 1.0), ivory black (value 0.6 to 1.0), Mars black (value 1.0 to 1.3) and vine black (value about 2.0).

Black can also be considered to be a perceptual component of other object colours, as in the NCS and in the attribute of brilliance (see blackness/black content).

Blackbody radiation

A blackbody or Planckian radiator is an "ideal thermal radiator that absorbs completely all incident radiation, whatever the wavelength, the direction of incidence or the polarization" (CIE 17-24-004). Such a body emits electromagnetic radiation with a spectral distribution that depends on the body's temperature. As the temperature increases this radiation passes from dull red through bright red, orange, yellow, white, and ultimately blue-white in appreance. The radiation emitted by actual heated objects, for example stars, approximates theoretical blackbody radiation. See also colour temperature.

Blackness/black content

In the terminology of the Natural Colour System (NCS) the blackness of an object colour is" the perceived amount of black in the colour relative to pure black" (NCS Glossary). In the Ostwald System the corresponding concept is called black content and was defined psychophysically rather than perceptually. See DOC Part 1.8. In both systems, object colours are considered to be resolvable into three components, blackness/black content, whiteness/white content and chromaticness/colour content. Black content also figures in defining the attribute of brilliance (q.v).

Colours are specified using blackness and chromaticness in the NCS and white content and colour content in the Ostwald system, specification of the third component being redundant.

Blue

Artists' paints classified as kinds of "blue" are characterized by predominantly short-wavelength reflectance and range in Munsell hue from near 10B (e.g. cerulean blue, PB35) to near 10PB (e.g. cobalt blue deep, PB73), with cobalt blue (PB28, near 5PB) commonly regarded as a middle blue. In additive technology, the blue primary ("B") in sRGB has a hue of 7PB.

As one of the four unique hues, blue is perceived to be a component of a wide range of hues in between unique red and unique green, including Munsell BG, B, PB and P. Different studies have found the average Munsell hue selected as unique blue to be 10B, 2.5PB and 5PB (Kuehni, 2012). In the NCS, hues from R10B to B90G inclusive have a blue component and the hue designated unique blue (B) is closest to Munsell 10B, about 2 Munsell hue pages from the conventional "middle blue" of painters.

Body colour

A body colour is a "color produced by absorption and scattering of light by colorants within a colored material" (ASTM E284-17).

Brightness

1. “attribute of a visual perception according to which an area appears to emit, or reflect, more or less light. NOTE The use of this term is not restricted to primary light sources” (CIE e-ILV 17-22-059). Note that brightness is thus a perception of the light coming from an area, in contrast to lightness which is perceived to be a property of objects. Brightness is the way in which we perceive physical luminance, subject to the brightness adaptation of the observer and visual phenomena including contrast and assimilation. See DOC Part 1.6.

"The adjectives "bright" and "light" principally have the same meaning, but usually have different applications: lights are described as "bright" or "dim", whereas surface colours are described as "light" or "dark". However, this distinction is not universal" (Note 1 to CIE e-ILV 17-22-061).

2. With respect to object colours, the term "bright" can signify freedom from blackness; thus whites, high-chroma colours and all tints in between can be described as "bright" in this sense. Cf. ASTM E284-17 "of an object color, combination of lightness and saturation". See ASTM E284-17 for additional meanings in the textile, paper and dyeing industry.

3. So-called 'brightness “B” in HSB colour space is a simple computation of brightness relative to the maximum possible for digital colours of a given hue and saturation.

Brilliance

- a scale of colour appearance along which related colours pass with increasing brightness from the black threshold through decreasing degrees of black content through a point of zero blackness (or “zero greyness”) to a fluorent (fluorescent looking) and ultimately luminous appearance. Brilliance is the way in which we perceive the brightness of an area relative to an expected maximum for a non-luminous object of its hue and saturation. See DOC Part 1.8.

Broken colour

1. Paint application displaying visible colour variants, for example obtained by incomplete mixing, as opposed to solid or uniform colour.

2. Paint colour reduced in chroma by mixing.

Brown

One of the eleven basic colour terms in English, brown refers to colours of orangeish red to orangeish yellow hue and moderate to high blackness. Kelly and Judd (1976) include in brown colours that are from 9R to 4Y in Munsell hue, and below 6.5 to 7.5 in value for colours below 6 in Munsell chroma, and below 4.5 to 5.5 in value for higher-chroma colours. Within this hue range they distinguish reddish brown, brown, yellowish brown and olive brown (see diagram under "earth pigments").

Brown, like grey, can only be perceived as a related colour; it is the way in which we perceive a light-reflecting object or a light-emitting object (for example, an area on a computer screen) when the spectral reflectance/distribution is biased towards the long-wavelength half of the spectrum and the amounts of those wavelengths are well below those in the reflectance/distribution from a similarly illuminated area perceived to be white.

“Chalky” colour

- in painting, colour felt to exhibit too much whiteness for its context in a painting (see also "muddy" colour).

Chiaroscuro

- Italian term literally meaning "light-dark." In painting, chiaroscuro refers to lightness contrast and is employed to depict the form of objects, to organize compositional structure, and to create expressive effects.

Chroma

Chroma is the chromatic intensity of an object colour (in everyday terms, the "strength of colour" of an object) and amounts to "the degree of departure of the color from a neutral color of the same lightness" (ASTM E284-17). The CIE definition of chroma as the “colourfulness of an area judged as a proportion of the brightness of a similarly illuminated area that appears grey, white or highly transmitting” (CIE e-ILV 17-22-074) can be understood by reference to the diagram below. Chroma is our perception of an object’s efficiency as a spectrally selective reflector or transmitter of light, and is perceived as relatively invariant under varying intensities of illumination (which changes its colourfulness). Chroma is commonly called saturation in contexts where different kinds of chromatic intensity are not distinguished.

Measures of chroma include Munsell chroma, C* in CIE L*C*h, and various predictors of chroma used in colour appearance models. In the NCS, chromaticness is a measure of relative chroma, specifying chromatic content as a proportion of what is deemed to be the maximum possible for the hue.

Chromatic aberration

- failure of a lens to focus all wavelengths of light to the same point, resulting in "fringes" of color along the boundaries that separate dark and bright parts of the image. This failure results from dispersion due to differences in the refractive index of the lens elements for different wavelengths.

Chromatic

1. A chromatic colour is a "perceived colour possessing hue" (CIE e-ILV 17-22-050). The CIE definition notes that in everyday speech the word "colour" is often used to mean chromatic as opposed achromatic colour, and the adjective "coloured" usually refers to chromatic colour.

The expression chromatic intensity (or in everyday speech, intensity or strength of colour) is suitable as an umbrella term for the degree of departure of a perceived colour from being achromatic, in contexts where chroma, chromaticness (relative chroma), colourfulness and saturation are not distinguished.

2. In the psychophysical sense, a chromatic stimulus is a "stimulus that, under the prevailing conditions of adaptation, gives rise to a chromatic perceived colour" (CIE e-ILV, 17-23-010).

Chromatic induction

"modification of the visual response that occurs when two colour stimuli (of any spectral irradiance distribution) are viewed side-by-side, where each stimulus affects the appearance of the other" (CIE e-ILV 17-22-013), or more simply, a change in the perceived colour of an area when placed beside areas of other colours. Also known as simultaneous contrast.

Chromaticity

1. Chromaticity is the colorimetric specification of a light independent of its intensity (luminance). A two dimensional specification is often appropriate for lights because for many purposes the colour of a light can be considered to be separate from its brightness. Examples of two-dimensional chromaticity diagrams include Newton's colour circle and the CIE 1931 x, y chromaticity diagram.

Chromaticity is defined in CIE terminology as the “property of a colour stimulus defined by its chromaticity coordinates, or by its dominant or complementary wavelength and purity taken together” (CIE e-ILV 17-23-052). An example of chromaticity coordinates would be the x and y coordinates in the CIE x, y chromaticity diagram. See also dominant wavelength and purity.

2. Sometimes seen used for chromatic intensity, perhaps in error for chromaticness.

Chromaticness

1. In NCS terminology, chromaticness is the perceived proportion of pure chromatic colour considered as a component of an object colour (along with blackness and whiteness). Chromaticness is thus chroma specified as a proportion of what is considered to be the maximum for the hue. Lines of uniform NCS chromaticness and uniform Munsell chroma have been found to diverge somewhat (see figure below). See DOC Part 1.8.

2. Outside the NCS, chromaticness is sometimes used as a synonym of colourfulness (ASTM E284-17).

Chromostereopsis

- perception of depth evoked by differently coloured areas on the same plane (Wikipedia). Most observers see red objects “advance” and blue object “recede”, although a minority see the reverse or see no effect.

CIE

The International Commission on Illumination or Commission Internationale de L'Eclairage (CIE) was founded in 1913 and is the organization responsible for the international coordination of lighting-related technical standards. In the field of colour the CIE established the fundamental framework of modern colorimetry including the CIE 2^o and 10^o standard observers, the CIE illuminants (Illuminants C, D50, D65 etc), various widely used colour spaces including CIE XYZ, CIE xyY and CIE L*a*b*, colour difference formulae (CIE DE94, CIE DE2000), and colour appearance models including CIE CAM02. Its International Lighting Vocabulary lists definitions for 1448 terms and is by far the most comprehensive and authoritative source on the scientific terminology of light and colour.

CIE 1931 RGB system

CIE colorimetry is ultimately based on colour matching experiments in which a light stimulus is specified by the amounts of three selected lights (called primaries) needed to match it. If one matches monochromatic wavelengths of light with any three real primaries, one of the primaries will often need to be added to the test stimulus to get a color match, in which case the amount of that primary can be regarded as a negative component of the match (below, left). Using monochromatic lights of fixed wavelength and power (CIE R, G and B) as the primaries, one gets for unit power of monochromatic lights of the spectrum curves as shown below right called color-matching functions (CMFs). A light consisting of a mixture of wavelengths can be specified by multiplying the amount of each wavelength by its colour matching functions and summing the results.

CIE 1931 standard observer

In many applications it is inconvenient to use negative values of colour matching functions, and so the CIE decided in 1931 to transform the RGB CMFs using a matrix transformation to CMFs that are always positive. These CMFs are non-real in the sense that they can not be physically realized (see Schanda, 2016). This set of colour matching functions defines the CMY 1931 standard observer.

As with the RGB CMFs, a light consisting of a mixture of wavelengths can be specified by multiplying the amount of each wavelength by its colour matching functions and summing the results to obtain its CIE 1931 XYZ tristimulus values.

The color-matching experiments considered by the CIE in 1931 were conducted with small, approximately 2 degree diameter color patches. If larger-colored fields are viewed or slightly off-axis objects are viewed, the 1931 CMFs do not hold anymore, so in 1964 the CIE standardized CMFs for a 10 degree visual field, the so-called large field CMFs (see CIE 1964 standard colorimetric observer).

CIE 1931 x, y chromaticity diagram

- a diagram representing the balance of long, middle and short wavelengths in a light from the point of view of the human eye, irrespective of the intensity (luminance) of the light. Two lights plotting at the same point in the diagram would be expected to match in colour, although the perceived colour of this matching pair would vary with changes in the viewing conditions. The diagram is based on the CIE 1931 XY,Z tristimulus values and plots the chromaticity of a stimulus in terms of x and y, the proportions of X and Y of the total of X + Y + Z (see diagram below).

CIE X, Y and Z are imaginary primaries lying outside the range of real lights that were devised in order to be able to specify all such lights using positive values. The chromaticities of monochromatic lights of the spectrum follow a horseshoe-shaped line called the spectral locus; this and the line of purples representing mixtures of the ends of the spectrum enclose the range of chromaticities of actual lights.

Although not the latest chromaticity diagram, the 1931 x, y diagram is still the most frequently encountered, for example in comparisons of the gamuts of different RGB screens and colour spaces (see additive primaries), and in diagrams showing the Planckian Locus (see colour temperature). In addition the important psychophysical measure dominant wavelength is still defined by the CIE in terms of the 1931 x, y diagram.

CIE 1931 XYZ tristimulus values

- values of three purely theoretical "primaries" that lie outside the range of actual lights and define a colour space CIE XYZ that encompasses all actual lights, and can therefore be used to mathematically specify all these lights with positive values.

CIE 1931 xyY colour space

CIE xyY is produced by combining the tristimulus value designed to correspond to relative luminance (Y) with the CIE x, y chromaticity diagram.

CIE 1964 standard colorimetric observer

- ideal observer whose colour-matching properties correspond to the CIE colour-matching functions adopted by the CIE in 1964 (CIE e-ILV 17-23-047). The color-matching experiments considered by the CIE in 1931 were conducted with small, approximately 2 degree diameter color patches. If larger-colored fields are viewed or slightly off-axis objects are viewed, the 1931 CMFs do not hold anymore, so in 1964 the CIE standardized CMFs for a 10 degree visual field, the so-called large field CMFs. To distinguish between values determined using the 2 or 10 degree functions, the latter are distinguished by the subscript 10, for example X₁₀, Y₁₀, Z₁₀ (Schanda, 2016).

CIE 1976 L*a*b* colour space

CIE 1976 L*a*b*, also known as CIELAB colour space, has three orthogonal coordinates, L* (CIE 1976 lightness) and a* and b* (chromatic coordinates corresponding roughly to reddishness/greenishness and yellowishness/ bluishness respectively). CIE L*a*b* was derived in 1976 from the CIE 1931 xyY colour space by a nonlinear transformation intended to make colour differences in the space more perceptually uniform. See CIE e-ILV 17-23-076. CIE L*a*b* is of central importance in professional graphics programs including Photoshop (where it is known as Lab space) and as a connecting space in colour management.

CIE 1976 L*C*h colour space

CIE L*C*h most commonly refers to an alternative formulation of CIE L*a*b* in which colour is specified in terms of hue angle h (measured counterclockwise from the positive a* axis) and chroma C* instead of a* and b*. This space is properly known as CIE L*C*h(a,b) to distinguish it from the corresponding space, CIE L*C*h(u,v), based on CIE L*u*v*.

CIE 1976 L*u*v* colour space

CIE L*u*v* has three orthogonal coordinates, L* (CIE 1976 lightness) and u* and v* (chromatic coordinates corresponding roughly to reddishness/greenishness and yellowishness/ bluishness respectively). CIE L*u*v* was derived in 1976 from the CIE 1931 xyY space by a linear transformation intended to make colour differences in the space more perceptually uniform. See CIE e-ILV 17-23-074. Unlike CIE L*a*b*, additive complementary hues remain opposite each other in plan view in CIE L*u*v*.

CIE illuminant, CIE standard illuminant

See illuminant.

CMY colour space (or model)

- theoretical colour space or model in which colours are specified in terms subtractive combinations of idealized cyan, magenta and yellow components.

CMY palette

- painter's palette consisting a cyan, a magenta and a yellow paint. Overall the gamut of a CMY palette is larger and more even than that of a traditional RYB palette, while the latter extends into higher chroma reds, oranges and yellows.

CMYK

- specification of printed colour in terms of cyan, magenta, yellow and black ink content for a given ink set and substrate.

Colorant

- "dye, pigment, or other agent used to impart colour to a material" (CIE e-ILV 17-22-076).

Colorimetry

- "measurement of colour stimuli based on a set of conventions" (CIE e-ILV 17-25-014). Colorimetry includes both visual colorimetry "in which the eye is used to make quantitative comparisons between colour stimuli" (CIE e-ILV 17-25-016) and physical colorimetry "in which physical detectors are used to make the measurements" (CIE e-ILV 17-25-018).

When we measure the colour of a light or object using colorimetry we are using the word “colour” in a specific sense based on colour matching (see colour, psychophysical). Stimuli with the same colorimetric specification will match in appearance in the same viewing environment (at least, to a mathematically defined “standard” human observer), but the perceived colour of these matching stimuli will be influenced by factors relating to the viewing environment.

Colour

The CIE International Lighting Vocabulary recognizes and defines two distinct senses of the word "colour": (1) perceived colour, corresponding to colour perceptions such as white, light grey, pink, red etc., and (2) psychophysical colour, meaning the measurable property shared by objects or lights that match in perceived colour when viewed together under the same conditions, assuming a "standard" observer.

1. Perceived colour is defined as the “characteristic of visual perception that can be described by attributes of hue, brightness (or lightness) and colourfulness (or saturation or chroma)” (CIE e-ILV 17-22-040). A perceived colour such as red or white or green is our perception of the overall spectral composition of a light or the intrinsic spectral reflectance of an object, in terms of its long-, middle- and short-wavelength components (properties that may be specified as the psychophysical colour). Perceived colour however depends not only on these spectral properties of the stimulus but also on "the size, shape, structure and surround of the stimulus area, on the state of adaptation of the observer's visual system, and on the observer's experience of the prevailing and similar situations of observation"(note to CIE e-ILV 17-22-040).

Perceived colours can be subdivided according to aspects of their mode of colour appearance, for example object colour, surface colour, volume colour, film colour, aperture colour, illuminant colour, illumination colour and proximal colour.

NOTE: The CIE ILV includes white, grey, and black as (achromatic) colours, but acknowledges that in everyday speech the word "colour" is often used specifically for chromatic colour, and that the adjective "coloured" usually refers to chromatic colour (note to CIE e-ILV 17-22-050).

2. Psychophysical colour is defined as a “specification of a colour stimulus in terms of operationally defined values, such as 3 tristimulus values” (CIE e-ILV 17-23-001). A psychophysical colour specification, for example a set of L*a*b* values, tell us what a stimulus will match, but the perceived colour of stimuli with this specification will vary depending on the viewing conditions. For example, areas of this screen and another screen having the same psychophysical colour specification should match in perceived colour when viewed together under the same conditions, but this matching perceived colour could be very different under different viewing conditions (for example, indoors and in sunlight). Psychophysical colour is also known as colorimetric colour (ASTM E284-17).

A psychophysical colour specifies the perceivable property of a spectral composition or reflectance. This specification depends in part on the visual system of the observer: for a trichromatic observer this perceivable property involves the overall amounts of three (long-, middle- and short-wavelength) components of the spectral distribution concerned.

3. The word "colour" is also used in everyday speech in senses reflecting the assumption that colours physically reside in objects or lights. When we speak of paints, or paint formulations, or spectral distributions including individual wavelengths as being colours, we are using the word "colour" in the senses that Green-Armytage (2006) identified as "substance colour", "formula colour and "spectral profile colour" respectively.

Colour appearance model

- mathematical model designed to predict various attributes of the perceived colour of a stimulus from its colorimetric specification and varying amounts of information about the viewing conditions, using various assumptions.

Colour atlas

"collection of colour samples arranged and identified according to specified rules" (CIE e-ILV 17-23-044). The colour samples are symbols of the colors of a colour space and illustrate the intended space only when viewed under the prescribed conditions by an average color normal observer (Kuehni and Schwarz, 2008, p. 381).

"Colour bias"

In those strands of traditional colour theory that promote the use of a "double-primary" or "split-primary" palette, the term "colour bias" is sometimes used for the secondary colour perceived to be mixed with the primary (for example a greenish yellow is said to have or to contain a green colour bias). This concept is sometimes conflated with the undertone or hue shift seen when a paint is thinned or mixed with white paint.

Colour constancy

1. Colour constancy in its broad sense refers to perceptual constancy of object colour, that is, the capacity of an organism to perceive an object as having a relatively stable object colour despite variations in environmental factors that may include the intensity and colour of the illumination, the background, and the intervening atmosphere. Colour constancy is generally imperfect to a greater or lesser extent; for example chromatic induction or simultaneous contrast is a partial failure of colour constancy with respect to variations in background.

2. The term "colour constancy" is sometimes restricted to perceptual constancy exclusively in respect to variations in colour of illumination, for example in the FSCT Glossary of Color Terms (1981): "relative independence of perceived object color to changes in color of the light source", and in ASTM E284-17: "the general tendency of the colors of an object to remain constant when the color of the illumination is changed", and is sometimes even taken to be a synonym of chromatic adaptation (e.g. Xiao in Luo (ed.) 2016, p. 281).

Colour harmony

- relationship of colours deemed to be aesthetically pleasing. Historically many colour order systems have been promoted as providing guides to colour harmony, but today colour harmony in traditional colour theory is primarily framed in terms of patterns of hue relationships.

Monochromatic. Combination of colours of a single hue.

Analogous. Combination of colours similar to each other in hue, for example two or three adjacent hues on a 12-hue traditional “colour wheel”.

Complementary. A combination of two complementary colours, variously regarded as the most "harmonious" combination through to the most discordant, or in more nuanced accounts (e.g. Rood, 1879), either harmonious or discordant depending on the chromatic intensities of the colours.

Split-complementary. A combination of a colour with two near-complementaries, often the two hues adjacent to the complementary on a 12-hue “colour wheel”.

Triadic, tetradic, etc. Combination of three, four or more evenly spaced colours, usually in the hue plane (though Itten suggested tilting the plane at various angles in his colour sphere).

Colour Index name

- a standard name for a particular colorant, consisting of the category, hue and identifying number. For example a particular green pigment has a Colour Index name of PG7 (Pigment Green 7). The names are set out in the Colour Index, a publication of the Society of Dyers and Colourists (UK) and the American Association of Textile Chemists and Colorists (USA). An extensive listing of artists' pigments by Colour Index name is available on the website http://www.artiscreation.com/.

Colour inversion

See discord.

"Colour mixing"

- Colloquial term for light or colorant mixture, including additive, additive-averaging and subtractive processes, carrying the unfortunate connotation that colours themselves reside and mix in lights and colorants.

Colour model

- term sometimes used (e.g. in Wikipedia) for what in CIE terminology are called non-colorimetric colour spaces (q.v.), for example a generic reference to "RGB" in which the R,G and B primaries are not colorimetrically defined.

Colour order system

A colour order system is defined by the CIE as an "arrangement of samples according to a set of principles for the ordering and denotation of their colour, usually according to defined scales", and usually but not necessarily "exemplified by a set of physical samples" (CIE e-ILV 17-23-017). Kuehni and Schwarz (2008) applied the term to colour categorizations ranging from those from antiquity through to modern colour spaces.

Colour rendering index

A colour rendering index (CRI) is a "measure of the degree to which the psychophysical colour of an object illuminated by the test illuminant conforms to that of the same object illuminated by the reference illuminant, suitable allowance having been made for the state of chromatic adaptation" (CIE e-ILV 17-22-109)

Colour solid

- "part of a colour space that contains surface colours" (CIE e-ILV 17-23-043).

Colour space

1. In CIE terminology a colour space is a "geometric representation of colour in space", ... "usually of 3 dimensions" (CIE e-ILV 17-23-041). A colorimetric colour space is a "colour space defined by 3 colorimetric coordinates" such as CIE XYZ or sRGB (CIE e-ILV, 17-23-042).

2. An alternative usage restricts the term "colour space" to colorimetric colour spaces and refers to non-colorimetric colour spaces as colour models.

Colour symbolism

- the use of color as a symbol in various cultures (Wikipedia). Most colours have a wide range of often contradictory associations, generally both positive and negative, even within a single culture.

Colour temperature

1. Colour specification widely applicable to lights, expressed in kelvins (K), in which reddish and yellowish lights have relatively low temperatures and bluish lights have high temperatures.

Colour temperature is defined by the CIE as the “temperature of a Planckian radiator whose radiation has the same chromaticity as that of a given stimulus” (CIE e-ILV 17-23-067), where a Planckian radiator (or "blackbody") is an "ideal thermal radiator that absorbs completely all incident radiation, whatever the wavelength, the direction of incidence or the polarization" (CIE 17-24-004) (see blackbody radiation).

Many actual lights are close enough to the chromaticity path of an ideal Planckian radiator (the Planckian locus) to make this concept of colour temperature applicable to them. The correlated colour temperature (CIE e-ILV 17-23-068) is the colour temperature of the Planckian radiator having the chromaticity closest to that of a given light in a particular chromaticity diagram (the CIE 1976 UCS diagram); lines of constant correlated colour temperature are shown in the diagram below.

2. Commonly used in traditional colour theory for the distinction between "warm" and "cool" colours/hues, in which the direction of increasing temperature is the opposite.

Colour theory, traditional colour theory

The term "colour theory" broadly refers to any body of knowledge relating to colour, particularly in the art and design field. Commonly divided into traditional colour theory, whose defining tenet is the historical view that there are three primary colours, red, yellow and blue, that all other colours can be mixed from, but which can’t be mixed from other colours, and modern colour theory, which incorporates more recent scientific developments. Much of the "colour theory" in textbooks written for college/tertiary level courses in art and design, as well as innumerable books and websites written for painters, designers and other colour users, is traditional colour theory in this sense. One of the most influential statements was Johannes Itten’s The Art of Color (1961) (see Briggs, 2018).

NOTE: The term "traditional colour theory" less commonly appears in texts with a variety of other meanings depending on which non-traditional theory is being contrasted.

Colour wheel

- alternative name for a hue circle, particularly in traditional colour theory where it is symmetrically structured around the three traditional primary colours, red, yellow and blue.

Colourfulness

Colourfulness is the chromatic strength (strength of colour) of the light reaching the eye from different areas of an object, as opposed to chroma, which applies to the colour seen as belonging to the object itself. A coloured object exhibits higher colourfulness (as well as higher brightness) in light than in shadow. Colourfulness also applies to colours of light transmitted or emitted by an object, and is the way in which we perceive the absolute amount of bias among the long-, middle and short-wavelength components of a light relative to daylight. See DOC Part 1.6.

The CIE definition of colourfulness is the “attribute of a visual perception according to which the perceived colour of an area appears to be more or less chromatic” (CIE 17-22-072). Note that the definition, in contrast to that of chroma, refers only to the appearance of an area and not to comparison with other areas.

Complementary colour/hue

- literally, the colour/hue considered to “complete” a given colour. The term "complement" was introduced by Thompson (1802) in the context of pairs of coloured lights that mix to make white light, referencing the idea that this mixture contains "all the colours of light in their just proportions". Thomson had observed that this complementary hue tends to be perceived in the shadows cast by a coloured light, and considered the pair of colours to be in "perfect harmony".

Pairs of lights that mix to make white light are now distinguished as additive complementaries, and the concept of complementary colour has been extended to other contexts including afterimage complementaries, paint-mixing complementaries, and the complementaries predicted by traditional colour theory. The complementary hue can vary greatly in these different contexts, especially near the yellow-blue axis. Very broadly, the additive complementary of a middle yellow (5Y) lies near its opposite Munsell hue of 5PB, while the afterimage and paint-mixing complementaries are more purplish and can be expected near 10PB.

Complementaries, additive

Hue of the light that will make achromatic (white) light when mixed with light of a given hue. Additive-complementary hues are the way in which we perceive opposite directions of wavelength bias in the spectral compositions of lights or the spectral reflectances of objects. Complementary lights have an opposite dominant wavelength on a chromaticity diagram, but because hue pages curve somewhat on a chromaticity diagram the concept of a precise additive complementary hue is inexact on a fine grained level. However additive complementaries are mostly less than one Munsell hue step away from the opposite Munsell hue. See DOC Part 4.3.

Complementary, afterimage

Hue of the negative afterimage of a colour stimulus. Although broadly complementary to the hue of the stimulus, afterimage hues tend to converge towards orange-red, yellow-green and violet-blue (Wilson and Brocklebank, 1955; Koenderinck et al. 2020).

Complementary, paint-mixing

The concept of complementary colour, introduced for colours of lights that mix to make white light, was subsequently extended to the colours of pairs of paints that mix to make an achromatic mixture, in which case the mixture is grey or black rather than white. See Part 7.5.

Complementary, traditional

Based on its defining tenet that all chromatic colours are mixtures of red, yellow and blue "primary colours", traditional colour theory holds that the complementary of any colour is the colour that "contains" the balance of its red, yellow and blue primary components, so that for example the complementary of a primary red must be a green that "contains" equal parts of primary yellow and primary blue. (The complementary of a primary yellow by this logic should contain equal parts of primary red and primary blue, which one might expect to be a magenta somewhere near 7.5RP, but in practice is usually identified as either "purple" or "violet". Typically these traditional complementaries are assumed to apply in any context.

Cone cells

Cone fundamentals

The cone fundamentals are the spectral sensitivities of the L, M and S cones measured relative to light entering the cornea. Cone fundamentals are more relevant to the response of the visual system than the absorbance curves of the cone photopigments or the responses of the cone cells in isolation, and compared to these they show reduced response to short wavelengths of light, due to filtration of these wavelengths before they can reach the retina.

Constancy (perceptual)

- "the phenomenon in which an object or its properties (e.g., size, shape, color) appear unchanged despite variations in the stimulus itself or in the external conditions of observation, such as object orientation or level of illumination. Examples of perceptual constancy include brightness constancy*, color constancy, shape constancy, and size constancy" (APA Dictionary of Psychology). [*The linked APA definition does not distinguish between brightness constancy and lightness constancy].

Context

Context refers to the setting of a colour stimulus, sometimes divided into the immediately surrounding background and the remainder of the viewing environment, called the surround (see stimulus).

Purves and Lotto employed the term "context" as a "general term referring to the information provided by the surroundings of a target; sometimes used to indicate juxtaposition in time as well as in space" (Purves and Lotto, 2003, p. 232), regardless of whether these surroundings consisted of a uniform background or an elaborate depiction of an illuminated three-dimensional scene.

Contrast

Contrast in the perceptual sense ("perceived contrast") is an "assessment of the difference in appearance of two or more parts of a field seen simultaneously or successively" (CIE e-ILV 17-22-089).

"Cool" colours/hues

- colours/hues deemed to have a psychological association with coldness. In different accounts the hue deemed to be coolest ranges from violet through blue to blue-green, and heated debates have broken out over whether a purplish blue is warmer or cooler than a greenish blue. See "warm" colours/hues.

Cyan

The word "cyan" is derived from the Latin cyaneus and the ancient Greek kyanos, meaning dark/blue. The modern connotation of a relatively greenish blue dates from Helmholtz (1866), who suggested renaming Newton's "blue" and "indigo" as "cyan-blue" and "indigo-blue" respectively. The name cyan is applied to quite different hues in different contexts.

1. In artists' paints the term cyan is commonly applied to paints based on the phthalocyanine pigments PB15.3 or PB16, sometimes with some phthalocyanine green (PG7) added. When mixed with white paint these paints mostly lie in the Munsell hue range 5B to 2.5PB.

2. The greenish blue colour of cyan printer's inks and ink jet printing inks, obtained using phthalocyanine pigments and dyes, and often called process cyan. The hue of this colour can be broadly correlated with Munsell hue 5B.

3. Among digital colours, commonly used for R 000 G 255 B 255, distinguished on this website as digital cyan. This blue-green digital colour (Munsell notation 6.6BG 9/10 in sRGB) is distinctly greener in hue than process cyan.

4. Commonly used to designate the blue-green optimal subtractive primary, which is similar in hue as digital cyan (roughly 5BG). Although cobalt teal (PG50) is closer to this hue, painters much more commonly use the bluer but cheaper, non-toxic and transparent phthalocyanine pigments as their cyan primaries.