Clean - Assuming that you are also using a clean sounding mic, this means that what is on tape should sound more or less like what is in the room. In other words, it alters the sound as little as possible.
Color - This term also is used when talking about microphones. Color just means that it alters (or colors) the sound in some way. Ussually not drastically, but changes the signal in some way, which may or may not be pleasing... but, it will sound a bit different than it did in real life. Warmth could be considered a kind of coloring.
Warmth - I'm not sure If I can describe this term well, as it is one of those qualitative things... But, warmth ussually means a smoother, rounder, fatter, softer tone... I think that for every person who uses this term, there is a slightly different oppinion of what it means. My best analogy would be the difference between a cheap solid state guitar amp, and a tube amp.... The tube amp sounds warm. Or... Elvis Presley compared to Sid Vicious... Elvis sounds warm, Sid, does not. Sorry if this is not the best explanation... It is a hard thing to describe.
These things can all be good or bad for vocals. It depends on how your vocals sound in the first place. If your vocals already sound drop-dead perfect, then maybe clean is for you. If you feel that it sounds thin, maybe you need to warm it up a bit.... It's a very subjective thing...
