I have to read all the posts in IMO and Breaking Mews, but, luckily, habitual color abusers are few and far between there, perhaps because only established members have posting privileges. I've found myself clicking "Quote" in order to read posts that are so light they're illegible in the original post, and then "Go back" on my browser. If I'm in a so-so mood, I find myself getting annoyed at having to take those extra steps.
I actually think "less is more", i.e., when somebody uses a color (preferably a deep one, since I use the Default Grey skin
) to just highlight a word or sentence, it makes their post clearer. On the other hand, when an entire post is in a bright color that "grates", or too light a color, I find myself skipping that post, unless it's one that has been reported. The overuse of color thus defeats the purpose.
The colored usernames/titles aren't as much of a problem, because they're limited to paying or former paying members, and I can highlight, or go by the avatar, siggy or location most of the time when I can't actually see the name. Glows or shadows might help those using IE, but don't show up if you're using Firefox, as Meaghan pointed out.
I've found the fonts can appear different depending on the browser being used, too. Laureen's appears very small on my laptop (the latest FF), but bigger at school (older version of FF) or on my PDA (PDA version of IE).