I found a new toy called Color Wheel Pro. The full lets you try out various color schemes (variations of monocromatic, complimentary, split-complementary, triadic, tetradic and freeform). You can even import your own images (in the form of special .swf files, which takes a little work but is worth the trouble.)
Here's some variations I did with my stars image. Interesting how changing the color schemes changes the mood of the picture.
p.s. Yes, I know there's no green or purple stars in real life. Too bad about that.