Colors, wavelengths, brains
Why do red and blue mix to make purple? When you mix two materials (of the same type), one of which reflects red and one of which reflects yellow, the resulting material reflects orange, a middling wavelength. Why is it that when you mix red and blue materials, the resulting material reflects something we view as purple/violet/thereabouts, rather than something middling, like a green-yellow? What is going on with the materials and what's going on with our visual perception system? Oh yeah, as someone pointed out this is only for subtractive color-things, but there's an analogous situation for additive color-things, and I'm too sleepy to state it. G'nite.