Although over time I have learned all the various color systems: Munsell, Yurmby, etc., I had no real use for this information until I read James Gurney's Color and Light: A Guide for the Realist Painter. If you want some practical color theory concepts that you can actually use in your artwork, I highly recommend this book. It has changed my otherwise dismissive view of color theory as Gurney manages to appeal to artists rather than making them want to stick their fingers in their ears and go, "La-la-la-I-cannot-hear-what-you-are-saying!"

In addition to color, I do pay attention to contrast and value, and color overtones (red, yellow, blue). At the start of a piece, I am interested in good composition and lights and darks which will pull your eye around a piece.

I also believe that learning about color mixing by trial and error is very useful. When you try to put a pale thick blue paint with a thin yellow, certain things are going to happen and it's good to know what those are before you waste a ton of paint! Take some time to mix your favorite paints with each other and see what happens. Those charts you can make showing color mixes are actually a very useful reference tool and are worth taking the time to build.

Color can be used in so many interesting ways and it can be enlightening to experiment. For example, I was sketching in the woods the other day and decided to do some small watercolors of trees using any color except brown. I got some wonderful bark texture from shades of purple and gold and turquoise, really unexpected and beautiful. That sort of playing around on a small scale can really improve your larger work and provide you with some new ways of looking at familiar subjects as well.