Of course, the basics of colour theory is taught to primary school aged children, and so everyone knows it; the whole circle of colours is made up of three primary colours (yellow, blue, and red), three secondary colours (green, orange, and purple), and six tertiary colours (blood orange, mustard, lime, turquoise, indigo, and magenta).
Hue is the professional word for what is generally known as colour. Red, yellow, blue, green - these are all words for different hues. The hue simply changes where on the colour wheel you are. Brightness is the word to describe how much black you add to the colour - the darker the colour, the lower the brightness. Colour language wise, brightness is fairly obvious; darker colours translate to darker themes, while brighter colours are easier to see and are therefore more thematically transparent. Saturation generally dictates how much pigment you add to a colour. The more "strong", or "bright" the colour is, the less saturated. Low saturation tends to have the opposite effect to low brightness; that being, coolness, calmness, and overall inoffensive and clear. If both the saturation and brightness are set to 0, then you get the default "corner colour". An infinite number of colours can be created by combining these three variables.
The one advantage of looking at all of the primary and secondary colours on a wheel is that you can take inspiration from what colours are next to each other and opposite each other; using this as a guide, creating an aesthetically pleasing colour pallet for your design is fairly easy, and can even tell the viewer more about your piece of art. For example, as a general rule of thumb, the colours between red and yellow tend to be "warm" colours, used to indicate danger, happiness, or vibrancy (depending on the context), while the colours between blue and purple are "cool" colours, and indicate peace, serenity, and sadness. Green and magenta are often considered "neutral" colours, with green generally indicating nature, and magenta being unnatural and vibrant.
One way to use the colour placement on the colour wheel, is to use "complimenting colours". A set of colours which are close together, often right next to each other, on the colour wheel can be used to create a dynamic colour pallet. For example, in the sketch to the top right, I have used a "yellow themed" colour scheme, and have used all of the colours around yellow as accent colours. the hair, which is the main focus of the design, is yellow, while the skin is orange tinted, and the bow is green. Using complimenting colours in your art can indicate coherency, peace, and simplicity. Another thing I have done with this drawing in order to convey these themes is use primarily muted colours. This is where I turn the saturation down, which makes everything look a little more cool and inoffensive. This can further push themes of peace and coherancy.
On the other end of the spectrum, colours placed on opposite ends of the colour wheel can also be used to convey themes. Take this sketch for example, (bottom right), where I have used the contrasting colours of blue and orange, which are directly opposite each other on the colour wheel. This, along with the fact that I made them incredibly saturated and vibrant, conveys themes of danger, unnaturalness, and high-energy. Using contrasting colours, however, can make a drawing look busy and offensive to the eye, and so I tend to reserve contrasting colours for accents (meaning small amounts of colour in accessories/eyes/small bits of clothing). For example, I might make most of the drawing comprised of complimenting colours, for example the yellow inspired drawing above, but make the eyes a saturated purple to add spice.
"Shape language" is a form of storytelling used in art, in order to subconsciously convey a message to a viewer by way of basic shape makeup. Different shapes can translate to different things when used in different ways and contexts. I have sketched examples of how this could be used in character design, as seen on the right.
Square shaped characters are often depicted as strong, stable, and reliable. Square shaped torsos, heads, noses, eyes, and general build can relate to maturity and masculinity, but can also imply that a character is boring. Examples of square-shaped characters in media are the Disney characters of Hercules or Gaston, who are both strong and masculine, or the dad from Cloudy with a Chance of Meatballs, who is supposed to be presented as boring and to-the-point.
Triangular shapes in a design can translate to many different things; danger, mystique, feminity, or even aggression. Many villains or love interests have triangles in their character design, for example Jafar from Aladdin, or Heather from Total Drama.
Circles in character design can imply many things based on the different uses and contexts; the obvious one, and the one most people think of, is obesity. While this is fairly common in media (for example, Ursula from the Little Mermaid), it is only one method of using circles to mean different things. Other uses could be round eyes or hair to mean innocence or cuteness, or just generally round features to mean childishness. Examples of this in media would be Steven from Steven Universe, or Anya from Spy X Family.