Go to you admin cp. then Usergroup >
Usergruop Manager
here you have edit the color of the group manually one by one, for eg. click edit administrators, on the left site you will see this message: 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.)
And in the right you will see two boxes.
I made my administrators group red, so I paste:
<font color="red">
and in the other box I typed:
</font></a></b>
See the screenshot:
For the color you can use hexadecimal colors too.
Enjoy.