system
Given the following post, respond as if you're commenting on Reddit:
human
Why are the psychological prime colors (red, blue, yellow) different from the mixing primary colors(magenta, cyan, yellow)?

Cobalt blue looks pure, niether greenish, nor purpleish, but the real primary color, cyan, looks like its just some random mix of green and blue.  Cadmium red looks pure red, but the real primary color magenta is violetish color. If I mix pthalo blue and scarlet lake, I don't get purple. I get a near black.  Is it training from childhood, or the way the brain works, or the color sensitivity of the cones on the retina, or some trait of light itself that causes this discrepancy?
gpt
Primary colors are defined relative to their application. The main gist is making the distinction of different ways wavelengths of light can be manipulated to absorb and reflect different colors. When a certain wavelength is reflected from a surface, it enters our eyes and we see the color. When a wavelength becomes absorbed by a surface, that color never reaches our eyes.

For the mixing of light like in old TVs and monitors, the primary colors used are red, blue and green and they are based on the premise that black is a complete lack of any wavelengths being reflected. The RGB color scheme is "additive", in that you start with black -- and mixing more colors increases the number of wavelengths that are reflected back to our retinas.

While for printing and pigment mixing purposes, cyan, magenta and yellow are used on the premise that white is the summation of all wavelengths of visible light. The CMY color scheme is "subtractive", in that you start with white -- and mixing more colors will *subtract* from the number of wavelengths being reflected into our retinas. After all, there is no such thing as a magenta wavelength... it is the result of green wavelengths being subtracted from white light.

So, the RGB colors are based on the premise of *adding* wavelengths to black to convey certain colors. By adding a whole buttload of wavelengths together (until they become pretty much a continuous spectrum), eventually *every* color becomes reflected and so what we see is white.

On the other hand, the CMY colors are based on the *subtraction* of wavelengths from white to convey certain colors by controlling which wavelengths are *not* reflected. So, eventually subtracting tons and tons of wavelengths means you will essentially absorb *all* colors, leaving you with black.