Not sure if it's just me, or if others have reported it, but I'm sure that during the recent Google layout changes the background colour of the ads in the top positions was still pale yellow. I'm now seeing a grey background. Looks like it might be being tested, because it's now gone back to pale yellow having been grey for a time.
You might remember in 2007 when Google changed the background colour from pale blue to yellow
[adwords.blogspot.com] in order to:
to improve the experience of our users and advertisers
For me the grey is a bit dull and makes the ads stand out much more, but only Google's stats will tell them whether it's a better alternative for advertisers, Google...and its users.