The vain of my artistic existence has been color theory. It isn't the fact that I don't understand it because I do. It has more to do with have the knowledge to confidently wield and bend colors to do my bidding. A step in the right direction is purchasing James Gurney's Color and Light book. The biggest "ah ha" moment I have had is the concept of Gamut Masking. In short gamut masking is when you apply a mask of over your color wheel and you limit your color choices to just that area of the color wheel. Obviously you can shift the lightness and darkness but you only chose hues that are visible in the map. I am just starting to experiment with this concept but it seems really powerful for creating some visually striking images. There is also a third party color picker (Coolorus) for Photoshop that can get you started with this concept.
Plus Coolorus has another nice feature called Luminosity Lock. It ables you to use different hues while keeping the luminosity between the hues the same. So if you are shading something you can pepper in different hues in the shaded area to give that area some more life. Hopefully that sense, if not, head over to Coolorus and download the 14 day trial and give it a go!