В шаблоне форум вид материалов.
Ищем $USERNAME$

И заменяем на

<?if($GROUP_ID$==1)?><font color="#F5F5DC"><?endif?>
<?if($GROUP_ID$==2)?><font color="#FF8C00"><?endif?>
<?if($GROUP_ID$==3)?><font color="#7CFC00"><?endif?>
<?if($GROUP_ID$==4)?><font color="#ADFF2F"><?endif?>
<?if($GROUP_ID$==5)?><font color="#00BFFF"><?endif?>
<?if($GROUP_ID$==6)?><font color="#00FF7F"><?endif?>
<?if($GROUP_ID$==7)?><font color="#FF00FF"><?endif?>
<?if($GROUP_ID$==8)?><font color="#FFFF00"><?endif?>
<?if($GROUP_ID$==9)?><font color="#87CEEB"><?endif?>
<?if($GROUP_ID$==10)?><font color="#FF00FF"><?endif?>
<?if($GROUP_ID$==251)?><font color="#FF7F50"><?endif?>
<?if($GROUP_ID$==255)?><font color="gray"><?endif?>
$USERNAME$</font>