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Originally Posted by linknoid
As a physicist (well not really, but I did major in it in college for a while), I would say that you're correct (at least in this comment, the next one where you say to throw out the spectrum completely is going overboard). In my explanation I specifically avoided going into the complexities of perception or even the details of what happens when you mix color. The point was to explain the basics of how colors work to someone who claimed not to know hardly anything about it.
Each person's eyes respond to slightly different wavelengths. In fact, some men only see 2 shades (they're colorblind), and some women actually, because of genetic issues, have 4 types of cones instead of 3, and they actually see a much different colors (and they're genetics mean if they pass on the genes that produce 4 colors for them, their sons will be colorblind).
But once you get past the issue of how each person responds to the various (and infinite) combinations of the colors of the spectrum, then you have to deal with where that combination of wavelengths and intensities are coming from.
First you have the light source. Each different source is different. The sun produces a relatively complete visible spectrum, resulting in very white light, but by the time it's passed through the atmosphere, a lot of the blue end has been scattered out, and even then the spectrum changes based on the time of day, the weather conditions, the pollution in the air, etc. And there are many other different light sources: incandescent lights, LEDs, mercury vapor lamps, halogen lamps, candles, wood fire, arc lamps, flourescent lamps, and the list goes on. None of them really produce a pure, even spectrum, and each one of the wavelengths it puts out interacts differently with different materials.
Then once it's produced by the light source, it has to deal with absorbsion (and re-emission as other colors), reflection, refraction, transmission, scattering, interference, and who knows what else. So when you mix two paints, you have to account for all those other things if you really want to know what color you're going to end up with. The thing the printing industry tries to do is simplify everything enough that they can reproduce most colors based on just a few, for practical reasons. Which is why we use primary colors. But that's definitely not the whole story.
So I hope you'll forgive me for make the vast simplifications to make the mixing of colors understandable and not overwhelming.
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