If you want your mail to stand out a little from the pack of messages invading your coworkers and clients Inboxes, you can always add a background color. Just be careful not to choose a color that is so dark it affects readability or so garish that it impedes your perceived professionalism.
If you just want to use a color wash for certain mails, simply add it the fly. Inside your Outlook message, go to the Options tab,click Page Color, and make your selection:
If you want all messages to be based on a common theme, in Outlook 2010 go to File, Options, Mail and click the Stationery and Fonts button:
Now click the Theme button:
This opens the Theme or Stationery dialog box, which offers previews of built-in options:
In Outlook 2007, look for these settings under Tools, Options, Mail Format. It is possible to make your own stationery, but that's a topic for a future blog post!