Additive and subractive color mixing

Paint is mixed according to the rules of subtractive color. When light hits paint, some wavelengths are absorbed, while others bounce off. A pigment that absorbs all visible wavelengths except red will be perceivewd as red, for example. If you mix two pigments together, they will both absorb their characteristic wavelengths. What you see is whatever light is not absorbed by either of the two pigments. For example, say you mix a greenish blue with a greenish yellow. The greenish blue absorbs everything but green. The greenish yellow absorbs everything but green also. So what you see when you mix them is green. Of course, any pigment reflects different amounts of different wavelengths, rather than one pure color. So even an orange yellow has a bit of green in it. If you mix the orange yellow with a violet blue, you will get a green. There will be much less green, however, so you will get a duller, darker green than with the first mixture.

Light mixes according to the rules of additive color. The results are different than those with subtractive color. If you mix a bunch of pigments together that among them absorb all wavelengths, you will get a black (or very dark grey). If you use a projector to mix all the wavelengths together additively, however, you will get white.

It's very userful to understand these factors. I once saw a painting depicting a sky shading from yellow to blue. The artist (an amateur) blended the two colors together and got a band of green between the two. Unfortunately, he was trying to depict an additive color mixture (light in the sky) using a misunderstanding of subtractive color. To depict such a sky with paint, you would never mix yellow and blue. Instead, you'd paint a band of neutral gray between the two colors. To avoid such mistakes, it's very helpful to understand these distinctions.