I didn't mean that you should follow the whole procedure for making a complex themable newsletter, you could just make a contemplate and wrap the content in a div so you have a div where you can put inline css inside for the background color.

For a colorpicker for Windows you could use http://www.colorschemer.com/colorpix_info.php
click the color in the email and search for it in your theme css.

simplenews_template module is not usable for Drupal 6 at the moment, there are some critical bugs in the issue queue. Also is rather a way to add a different header and footer per newsletter, but it has a place where you can add css, I'm not sure if this will be more effective than changing mail.css though.

Hans