Old Yellow Color:

New Pink Background:

Google has said that this change is part of their recent changes to the search results page and will be rolled out "100% globally by the end of this week". The also pointed out that this is purely an aesthetic change that won't have any impact on the way they target or serve ads.

This is not the first time Google have changed their AdWords background color, back in 2007, Google changed from a light blue color over to the yellow background displayed above.