I think guests names should be a different color when you check who is online . I Have had times where this would have been helpful.
I don't think this is possible to do, however I do agree with you. Most of the time I get on I can't tell if the people online are guests or just people I don't know and not having a way to tell the difference without waiting for someone to talk is kinda annoying.
We once had seperation of all of the ranks in /who back when we had yellow names Admin: Mod: VIP: Citizen: New: Guest:
This might work http://forums.bukkit.org/threads/chat-simplycolor-v1-0-simple-plugin-for-colored-nicknames-1-2-4-r-1.28611/