Red + yellow = orange. Orange sits between red and yellow in the rainbow. It makes sense mixing them provides an “average”
Yellow + blue = green. Green sits between yellow and blue in the rainbow. Same as above.
Purple does not sit between red and blue in the rainbow. Why do they combine to make purple?
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The same reason we can’t distinguish yellow light from mixed red and green light. The eyes don’t tell the brain what wavelength light it’s receiving, just the intensity of light triggering the different receptors. Sometimes you get red and blue light without green light, and purple is how you interpret that input.
If we had eyes more like a mantis shrimp or spectrometer, we might interpret colors as wavelength, be able to tell the difference between yellow light and mixed green and red light, or be able to tell that purple is really red and blue mixed together. Or we might just see more distinct colors and not inherently know about wavelength.
There are people with four types of cones, but in order for everybody’s eyes to work that way you need some strong selection pressure benefiting people that can see more colors. For example, if that was the only thing that let you distinguish a poisonous plant from a nutritious plant during a famine. The people that could tell the difference would have an advantage and more of them would survive.
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