With vBulletin, administrators can make "custom fields" in profiles, which can be used with the advanced template system. I created a custom field called "Username Color" and used it in the "postbit" template, which is the template for posts. However, I set it so that only administrators can change the field.
Code:
<normalfont color="$post[fieldX]">$post[username]</normalfont>
(where X is the field ID)