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Colour Codes are created to sample all the distinct colours in an image for reference and matching in a 3D model.
If you are using a Mac computer, there is an app called "Digital ColorMeter". If you don't have a Mac or your Mac doesn't have this app, you can use any program that gives you an RGB reading.
If you hold your cursor over a tile in the colour code, it will give you a R value, a G value, and a B value. These stand Red, Green, and Blue respectively. Copying this information, you can head over to Tinkercad and input this data. My colours:
R: 101
G: 96
B: 78
You might want to write these down when you have them.
Select your shape, click on the colour circle, choose custom colours, show the color values, and input your data, Red Green Blue (R, G, B) respectively. This gives you your identical colour match! You can match objects in the image with the objects in the Colour Code and use this to match your colours directly from image to model!