Color theory, as found in most art books and art classes, doesn't actually help a working painter all that much. You may find that whenever you try to mix a specific color, you get "mud." You might cope by just getting a lot of tubes of paint so that you rarely have to do much mixing. Seeking clarity, you might buy a book like "Blue and Yellow Don't Make Green," which promises a new approach to color, but is based on concepts invented in the 1700's. (And written in an irritable, pretentious, finicky style. By a guy who doesn't know how to construct grammatical sentences. But I digress.) Or you find something like the Munsell color system, which does a good job describing color, but doesn't show how to mix those colors after you identify them. Reading books and looking around on the internet gets you a little closer, but mostly, by trial and error, you just figure out what works, using a small subset of available pigments. You memorize some useful mixing recipes. A lot of the time, you muck around with paint until you get something that looks about right.
If you delve more deeply, you find that the subject of color is incredibly complex, because it requires reconcilliation of the physics of light wth the messy, non-linear neuroanatomy of the human retina, optic nerves, and visual cortex. Most of what's written about color is not for painters, and most of what's written for painters is by people who've learned to mix paint, but don't actually understand color that well. One excellent resource is the very fine handprint web site, where the author has done incredible amounts of reading, research, and testing with watercolor paints. But the stuff he has on color goes on and on, and on and on, so it's hard to find the real practical stuff (it's there, and it's worth looking for).
So, while I don't pretend to have a really thorough understanding of color as it pertains to painting, I thought I'd try to boil down what I do think I have a clue about. It's a little easier for me, since when I was in graduate school I did a bunch of work with the psychology of visual perception (I'm even published in the field). I will not, however, subject you to complex equations, the details of opponent process color vision theory, or technical color space specifications that are designed to meet the needs of the print, computer monitor, and motion picture industries (you're welcome). I'll try to stick with what you need to know in order to describe and mix colors.
So, to start out, we need to dump the color wheel. It was a useful innovation back in Isaac Newton's time, but we've moved on since then. The biggest problem with it as a tool for painters is that it's trying to do two different things at the same time, and it does both of them poorly. First, it tries to provide a model of human color vision, including how the eye processes complimentary colors—whatever those are. But when you test how actual vision works, you find that the color wheel is a terrible model of color vision and that much more accurate models have existed for well over a century. Second, it tries to provide a guide to color mixing. It does that very badly as well, because real color mixtures don't fit the standard color wheel model in any coherent way.
It's become apparent to me that we must divide the topic of color for painters into two: (1) a way to describe color as it is found in the natural world and as the eye perceives it; and (2) a way to conceptualize how to mix desired colors using particular combinations of paints. There is no system that does both of those tasks, so let's just dispense with the color wheel and start over with two separate (albeit related) topics. And we'll get to those topics in later posts. I promise.