[vertigo jones]

Admincp - > Usergroups -> Usergroup Manager -> Edit the appropriate group ->

There are two fields called:

Username HTML Markup
Specify an opening and closing HTML tag with which to display the names of users in this group. This applies to names that appear on Logged-in Users, Who's Online, Users Browsing Forum, Users Browsing Thread, Posts, Member List etc.

So you could do something like:
<span style="color: something;">

in the first

and then:

</span>

in the second.