Per a request in a private message, I have changed the Administrators group's custom username color from #AA0000 (Like This) to a brighter red #FF3300 (Like This), which should have better contrast with the post background.
What other color changes need to happen to make forums more usable?
This is a good change. In general, for color-blindedness or not, having important details differentiable by luminosity helps readability for everybody. I don't have any color-blindness, but the dark admin text was a little hard to read anyway.