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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Your eyes have three types of color receptors – what we’d call Red, Green and Blue.
The level of activation of each type results in what we perceive as colors. Some wavelengths of light activate multiple types of receptors, resulting in blends that we perceive as mixes of the three primary colors.
As you go off the blue end of the spectrum you detect blue, the green receptors mostly don’t pick up anything, but the red ones have a small uptick in sensitivity [graphical representation](https://midimagic.sgc-hosting.com/eyespect.jpg).
So you are detecting “red” at the very end of the “blue” end of the spectrum, giving a warp-around effect to your color perception, which makes purple appear to be between blue and red.
Edit: [A lot more info here, with nice accessible graphics](https://midimagic.sgc-hosting.com/huvision.htm)
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