Navigate the the location below in your registry and make the corresponding change.

Key:[HKEY_CURRENT_USER\Software\Microsoft\Internet Mail and News]
Value Name: ColorCycle
Data Type: REG_SZ
Data:(0 = disabled, 1 = enabled)

Open a new message in Outlook Express and press hold "Control" and "Shift" and press "Z" to see the available backgrounds.

 

Back to Outlook Tweaks