I want guests' names to show up coloured and bold, like users in other groups.
I go to ACP and set up Guests color as black (000000). I go back to the forums and actually see what I wanted (except that a little extra space - padding - is added before the name).
But now, when new guests post new messages, their names in index and viewforum are displayed gray and non-bold (while they're still black in viewtopic).
If I go back to the ACP, unset and then set again the group's color, the situation is fixed, but again if new guests post new messages the issue reappears.
In other words, color setting for the Guests group applies only to already existing messages (both in prosilver and subsilver).

I solved the problem modifying the template, but it's quite annoying.