Colors are okay sparingly, but people use various forum themes and some of them make colors annoying to read. I wouldn't worry about that though. As long as your entire posts aren't in a different color, they're okay to use for special reasons.
For example, I sometimes use small white text to denote a significant time change. Caden Law used it to illustrate his "wizard voice". Although I think him doing that looked a little weird, since all his characters had different-colored wizard voices and I couldn't take them seriously...