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.

