When I create an ad with a light green background (#F5F5F5) and paste the code onto my pages the words "Ads By Yahoo!" are white. You can barely see the words and there is no way for me to change them.
I do not want to break the Terms and it seems like Yahoo has chosen the color of their words to be white (even on a near white background).
I had the same problem with using a light-brown border. I emailed Yahoo about it, they sent a canned response and it was never fixed. You'd think they'd care about this but apparently they don't.
I've had the same 'problem' from Day One. I didn't contact them about it. The more transparent they are, especially in light of this rotten targetting - the better I like it.