I know there exists a mod to do this for you;
http://www.phpbb.com/mods/db/index.php? ... ib_id=1639
however, you can also do this by editing one of the files in your template's folder via FTP. Unfortunately, I cannot help you to that extent
I haven't personally tried the modification on my forum, but it looks as if it will do exactly what you want.
There's also a mod that allows you to color different usergroup's members (similar to phpbb3 if I'm not mistaken):
http://www.phpbb.com/mods/db/index.php? ... ib_id=1685
Hope this helps; I don't know if you were looking for a mod or a tutorial on which files to edit, but there are some good mods for you anyways