Pretty much what Herr Gentleman of Colour said. I know it's in his sig, but here's a link to his very informative website about color.
As for my input, I've found that learning to paint with watercolor in glazes is excellent practice for using and understanding layers. Because you have to account for how each glaze of color will interact with the last and how the value darkens with each glaze, it helps understand how you can't just directly slap transparent color over a grayscale image and expect perfection.
I wouldn't necessarily suggest taking up watercolor as a serious medium, but picking up some paints and paper to do color charts and value scales via glazing would likely help. But if you're not so inclined, you can also try that out digitally.