Color Theory was one of my favorite classes in design school. It was also probably the most difficult class I've ever taken (well... most difficult class that I cared about).

Ultimately I think color comes down to an instinct, you either have good ideas about it or you don't, though maybe the most important thing is willingness to try. A lot of good ideas come out of just trying things and making a decision. Still, there are a lot of theories behind color that are useful to know as a designer, if only to be able to clearly explain what you want.

This actually has very little to do with all of that, but it's a fun game if you have five minutes to spare: test your color IQ.