While I can see your point, personally I like being able to set the colour of a nick to be the same as the background. I use this setting (although I might be the only one), to make idle users "invisible" in the nicklist. (or at least, presuming I'm understanding your suggestion correctly, which is, to say, that the colour of a nick be shown against the background, so that if someone was using a black background, a nick coloured black would show up as a different colour).
I hope that makes more sense than it does to me now, re-reading it. LOL