Howdy! This is one of the easiest things in vBulletin - just posting this for our beginners
Go to AdminCP > Usergroups > Usergroup Manager
Edit usergroup. You have to do this for each usergroup that you like to be colored.
Add this code in username HTML Markup:
<span style="color: XXXX;">
Replace "XXXX" with your color name/code, visit http://www.w3schools.com
for a HTML Color Table.
Save it and you're done!