Go to your AdminCP. Expand the Usergroups section. Click on Usergroup manager. That should list the usergroups in the main frame of the CP. Click on "Go" next to the Admin usergroup. In the fields for "Username HTML Markup" (it's the fourth field down from the top) put the following code in the first box:
Code:
<span style="color:blue;">
And in the second box put:
Code:
</span>
Go to the bottom of the page and click on "Update".
Go to the moderator usergroup next and do the same thing except change blue to orange.
Done.
The default color is blue (from what I recall) so you may not see a change with the Admins but you should with the moderators.
Now go to your forum, hit refresh on your browser. They should appear the way you want. If not, clear your browser's cache and refresh the page again.
edit: I just noticed that the previous codes that were provided used "font-color" whereas just the word "color" worked for me. I don't know if that makes a difference but it could.