Here's an example for the admin usergroup...
In adminCP go to usergroups==> usergroup manager
In the line that says 'Username HTML Markup ' go to the first box and enter
HTML Code:
<span style="font: 16px verdana; text-decoration: underline; font-weight: bold; color: red;">
In the second box enter
You can change the stuff in the first box to your liking such as change 'red' to 'blue' or remove the part about 'text-decoration: underline;'
Follow the same procedures for the other usergroups