No, you can not alter the code. But there is a way: Use a php include that returns 50% of the times one google adsense javascript code and a different one the other 50% of the times. Then use channels to keep track of it if you want. That way you could use up to 8 different colors.
You can use more then 4 colors although Im not aware whether that would be against their TOS. You can use formats like : google_color_url = ["FF0000", "008000", "15548A", "FF8000", "FF0000",]; Code (markup): Im not sure whether this works for me its been like almost a year ago since I have tried using multiple colors. You could use a php rotating script as well.
Lazy bastard. Whatever. Shows what kind of person you are. Anyways, who said I didn't read the TOS (or is it TOC, lol). I can't seem to find the specific passage where this is stated specifically. Just made me curious.