After migrating from Entourage to Mail.app, I really missed the Entourage feature that emails from a contact belonging to a category (group) get the same category (group) color as the contact. But this is an easy task to do with Mail.app's rules.
In my case, I created rules for every group in Adress Book, with these criteria:
Sender is member of group - [group name]
Action: Set Color - background - [color to use]
Unfortunately, Mail.app only allows one to set either the background or the text color. Therefore, dark background colors are a bad choice as you won't see the black text anymore.
Another helpful rule is to set all emails from senders who are not in the Adress Book to a different color -- I chose light gray for the text color. Now I already have an idea from whom I receive an email without even reading the sender, and know at once if someone changed their email adress, which I haven't yet added to my Adress Book.
Of course, you can colorize your email after receiving, too, by selecting all the email and choosing "Apply Rules" from the contextual menu.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050709122514104