Google AdWords is like that kid who is always looking for attention. Every few months they change their hair color or dress style. Google has decided to go back to the yellow color only about 6 or so month after switching to purple.
Google confirmed the change telling me, “I can confirm the change, which should be fully rolled out globally by the end of this week on Google.com as well as our local domains,” Google told us. Google added, “this is purely an aesthetic change and won’t have any impact on the way we serve advertisements.”
If you see purple still, enjoy it because it is going away very soon.
The history of Google AdWords background colors:
February 2011 - Back To Yellow:
July 2010 - Pale Purple:
April 2007 - Yellow:
Prior - Blue :
Test of Green May 2008:
I assume the color changes ever so often is to get their searchers who are accustomed to certain colors to notice the ads. Ad blindness also applies to search ads and changing colors helps reduce that blindness.
Forum discussion at Google Web Search Help.