I saw this as an option yesterday.... but not today. Maybe I'm just missing something. I could have sworn I saw an option to have multiple themes per ad code. Where did that option go?
http://adscience.medspan.info/2005/09/13/how-to-change-colors-the-chameleon-code-for-google-adsense/ Look at that site.
Just press ctrl and select all color themes you want to use for that ad. Then Google show random color theme for that ad every time somebody open your page...