A Cre8asite Forum thread named Google is testing a new Adwords format shows how the top three sponsored ads in Google SERPs are showing up different shades of background colors for one specific user (at least).
The screen capture above is from the user reporting the new sighting. It is very possible that this is an other UI test by Google. But this can also be a spyware or adware attack. No confirmation from Google as of yet.
Update: Official Response from Google (Thanks Gary Price):
Looks like our older UI. We used to do ads in different (tasteful) colors. Note the old version of the tabs for (Web Images Groups Directory News Local). I suspect that this is just a screenshot from years ago. The tabs wouldn't look like that on a current UI test.