Looks like Yahoo is now experimenting with their own background colors for search ads. Spotted by @tecnonetblog - Yahoo seems to be testing a green background color for their ads. Personally, I see a light blue colored background for Yahoo ads.
Here are screen shots of the test.
Green Background Yahoo Search Ads Test:
Blue Backgrounds Yahoo Search Ads:
Google is frequently testing different colors and shades of backgrounds for their search ads, so this should come as no surprise.
Forum discussion at Twitter.
