Aside from the classic Librus Cyan? Hmm! I guess I'd have to say I like any and all colors when used in a good, thematic role. A monochromatic palette accompanied by colored accents can really add a sort of "vibe" to something, don't you think?

Aside from that, I quite like monochromatic hues. Oooh - and you can't ever go wrong with a little bit of a nice, golden trim on something.