The latest release of Yahoo! Mail Beta, now included a new feature -- an ability to add stationery (background colors and image templates) to your messages. Stationery is a feature that existed in the previous version of Yahoo! Mail. Since the release of Yahoo! Mail Beta.
To use Stationery, go to the Applications area of Yahoo! Mail and click on the application named "Stationery." Once you click on Stationery you can then choose the background you wish to use in your message. Select from the drop-down menu and click on the background image of choice.
After clicking the background image, a new message window is launched. From there, simply type your message and send it along with your favorite background applied.
[Source: Yahoo Mail blog]