[Damian]

here is a way to do it for the Admin usergroup as an example.

Add something like this to your Additional CSS Definitions:

/* ***** username colors ***** */
.admin_username {
font-weight: bold;
font-style: italic;
color:#FF0000;
}

Then:
AdminCP --> Usergroups -->Usergroup Manager --> Edit Administrator Usergroup --> Username HTML Markup --> Add this for opening and closing HTML tag :

Opening: <span class="admin_username"> Closing: </span>

Do this for each usergoup you want to color usernames. The advantage to doing it this way is you can define different colors for multiple styles if you have them installed on your board.