I may have a thing about GW for it's overpriced (IMHO) paints that always dry up on me, but I am in awe of the general standard of Fantasy/Sci-Fi miniature they inspire. Hence the purchase of a rather expensive painting guide, but I believe it to be worth every penny.
If you see one in a store then turn to page 34 and read the brief section on Colour Theory. It's worth the price just for that (again IMHO). Not just useful for Sci-Fi and Fantasy, but applicable across all genres and scales :)
For me it is especially useful for "washes", "inks", reinforcing how "complementary colours" make layering (shade, mid and highlight) make, mixing paints and opening my eyes as to how "contrasts" work. Lots of other stuff tucked away in there too.