Got any examples of these colours so we can see the contrast? Orange or blue seem like they could work.
I have to say though, I'm a fan of the normal white. Titles or tags look fine, but something about seeing different coloured usernames makes me read the post differently, and I honestly can't explain what I mean by that. If it's different, it should be very subtle. A light pastel blue or something along those lines at least. Just something that doesn't stick out in your peripheral vision while reading a post.