Since *purplekecleon's Color Tutorial hit the big time, I've been paying attention to color theory when browsing dA.  One of the things I've noticed now is that a decent handling of color can change a piece from having an infinite list of flaws, to looking artistic.  In contrast, with black-and-white, there is only value to work with, so a piece needs to have an excellent rendition of shape and shadow to appear as a nice artwork.  When shape, value, and color are all in place, especially with a high level of detail, that's when the image looks downright stunning.

I've also decided that one of the best ways to pick colors is to include a palette layer, with splotches of base colors on it.  (Obviously, this layer is not visible in the final image.)  Then, take the color you want to shade to, and lightly paint over the base with it, and take a colordrop of the result.  Also, it might be helpful to add another blotch with this new color on the palette layer, for future reference.  Finally, go to your desired layer and paint with the base and highlight colors.  (That italicized bit is the hard part.  I really hate painting on the palette layer in MyPaint, because it doesn't have selections to cut/paste it into the proper layer.)