go to Admincp>Usergroups>Usergroup Manager>
-edit the usergroup you want to change color, in there you have this:
Username HTML markup, in first section put: <font color="Red"><b>
in the other: </b></font>
for each usergroup do the same thing but with other color,
you may put the colors in HTML code like: #53c720
example: for the word red you may put the html code witch is #FF0000
if you dont know the codes for colors, just google the "HTML colors code"
cheers!!