[nitrogen]

A large number of people can't tell ads from search result, so the difference isn't that big in in practice.

In some cases this can be caused by the background colors chosen for ads being perfectly eliminated by cheap laptop LCDs at slight vertical angles. The pale yellow or blue completely disappears to white. I wouldn't be surprised to learn that A/B testing of colors stumbled upon the perfect color to disappear into a cheap LCD screen ;-).