I think the issue is that you normally try not to add queries because they take up time. The user table is already joined in that query to get the info for the last poster, which may be why there's not usergroup coloring for thread starters by default. Anyway, you could just add a query at threadbit_display to look up the info, but then you're adding one query for each thread. The other thing you could do one "big" query similar to the one that's being done (I'm looking in forumdisplay.php), maybe at forumdisplay_query. I don't know which one's better, it probably depends on a lot of factors.
Anyway, if you didn't care at all about adding queries I think you could do this at threadbit_display:
Code:
$starterinfo = fetch_userinfo($thread['postuserid']);
$thread['starter_musername'] = fetch_musername($starterinfo);
Then you could use {vb:raw thread.starter_musername} in the template. That assumes that what you want is to have the username colored by group in the same way that it would be in other places. (I haven't tried the above code).