Here's a good tutorial to check out with regards to color: http://www.deviantart.com/art/Tutorial-17-A-turtle-walkthrough-345468329 (Cyril Rolando is a really talented colorist & digital painter, and in my opinion an excellent study)
If you're having a lot of difficulty grasping color, though, it may be a symptom of another difficulty you have. If, for example, you don't have a solid grasp on shading - your colors are always going to look flat, lifeless, grade-schooler 'color in the lines' kind of stuff. In which case, rather than practicing color, practicing shading may be better in terms of your advancement.
If you happen to be an artist who does most/all of your shading using opaque black & white over solid color? Then it's probably worth looking into studying, as you need to have a good grasp on light & shading before you can solidly move on to color manipulation. For shading practice, I'd suggest you practice painting still life/realism pictures (like the one TristanJensen provided for study), in some very basic values (black, white, light gray, dark gray).
I don't know your art or your exact situation, so I figured I'd just put that tidbit out there.
Best of luck to you & your studies! I hope I helped. 
