Found a tip on this site: http://deadline.3x.ro/adsense/adsensetips3.html Where it shows how to add an array of colours to be used with the Adsense code like this: google_color_border = ["628098","628098","000000","000000"]; google_color_link = ["000000","BBB90C","BBB90C","628098"]; google_color_url = ["000000","628098","000000","000000"]; google_color_text = ["628098","628098","000000","000000"]; This constitutes modifying the code, does it not? If it's not against TOS then I definitely would try it.
I think if you want to randomize the colors you need to use an external script so it shows up normal in the HTML page. Such as using php to get a random color.
Google is already offering it mates. You can pick up-to four different patterns for your banner. Just open your adsense account and get the new code by using CTRL+Click combination to select more than one color for your banner.
Cool! I couldn't do it using the Google control panel but since you've said that it's available from them, I guess it's ok to use.