RGB is the color scheme whose primary colors are red, green, and blue. The reason for this color combination is a matter of our human biology, and not some underlying physical principle. Our eyes simply have receptors that are mostly sensitive to those colors, and so this maximizes our responsiveness to this color scheme. Most importantly, this is an additive color scheme, which means that we are starting from black (no photons) and increasing the number of photons that reach our eyes when we add these colors into the mix.

If you were to take a very close look at your computer monitor or cellphone screen, you would see that each pixel is constructed from three subpixels. Unsurprisingly, the colors of those subpixels are red, green, and blue. The way that colors are produced is that those subpixels display at different levels of intensity. The itensity levels are usually given as a number from 0 to 255, where lower numbers mean lower levels of intensity. (If you have ever seen hex color codes, such as #A72EBD, you have seen this color system. The only difference is that the numbers are being represented in hexadecimal instead of using our usual decimal number system. If this were a math class, we’d talk about how hexadecmial works. But it’s not, so we’ll just direct you to search for a hexademical to decimal converter online.)

The ideas for this system go back to the 1920s when Technicolor developed their three strip system. At that time, film was only able to capture black-and-white. How can you use black-and-white film to capture color? The basic idea was to capture the footage three times, with one film capturing the reds, one film capturing the greens, and one film capturing the blues. Then the three layers can be put back together using red, green, and blue lights to reproduce the original image.

CMYK is the color scheme whose primary colors are cyan, magenta, yellow, and black. This is also built around human biology, but moving in the opposite direction as RGB. Instead of starting with black and turning up the colors, this starts with white and removes colors. This is used for pigment-based colors, like paint and ink, where the illumination source is external and the colors that are seen are only from reflected light.

The reason a white piece of paper looks white is because it’s reflecting red, green, and blue photons back at us. If we add red ink to the paper, the reason that we see red is because the ink is absorbing the green and blue photons, and is only reflecting back the red ones. And this is what is meant by subtractive color. However, when it comes to pigments, thinking in terms of red, green, and blue turns out to be more complicated when you mix them. The underlying reason for this is that generating red means absorbing green and blue, and it makes more sense to have the primary colors absorb single colors rather than pairs of colors.

And this is where we get the cyan, magenta, and yellow base colors. Cyan is the color you get when you absorb only red light. Magenta comes from absorbing green. Yellow is the result of absorbing blue. Then you can mix two of these colors together to get the secondary colors of red, green, and blue. For example, red is a mixture of magenta (which absorbs green) and yellow (which absorbs blue).

The role of black in this system is because there’s a gap between theory and practice. In theory, if you mix cyan, magenta, and yellow together, you should get black. However, in practice, the pigments are not perfect. That is, cyan doesn’t absorb all of the blue, and so on. This means that mixing the colors together usually results in a brown color. So adding black as a separate color sidesteps the challenge of needing to make perfect cyan, magenta, and yellow colors in order to mix them together to get black.