system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If red, blue, and yellow are the primary colors, and all colors can be made from them. Why do we use red, blue, and green on computers to make colors?
gpt
Technically, the primary colours of paint are cyan, magenta, and yellow (CMYK). These colours are the exact opposite (complimentary) colours of red (opposite cyan), blue (opposite yellow), and green (opposite ~~green~~ magenta). 

RGB are the real three colours of light that our cones can react to. Short, medium, and long cones react to blue, green, and red light respectively. Anything in between these three colours will activate the different cones to different degrees, and our brain will determine the colour we see based on the combination of reactions.

The idea behind CMY(K) is that you're absorbing the colours away from white light, hence reduction. Their combinations will result in the opposite effect of adding real colours of light. Combining cyan and magenta will provide a blue colour, because cyan absorbs all red, and magenta absorbs all green, leaving behind blue light being reflected. This only holds true if you're viewing the colours under white light however. If you looked at it under yellow light, then the colour you would see off a cyan/magenta ink mixture will be black, as there is no blue component reflected.

Of course, if you're trying this with acrylics, the colour might look muddled because of an aspect of paint called chroma. Generally, the more colours you mix, the more hue/colour you would lose, and it is a fundamental idea behind paint mixing.

Your idea of red, yellow, blue primary colours is really just an approximation of cyan (close to blue), magenta (close to red) and yellow, but they serve their purpose in reaching many hues on the colour scale.