An eye tracking study for AdSense ads by Tobii suggests that more information is remembered when it has a high contrast color. “Increased awareness can lead to higher recall of the ad, thereby, helping build the brand of advertiser which can increase clicks.”

This is a little confusing because if people are remembering the ad they presumably will later come in directly – without clicking since they won’t be looking at the ad.

This result is seemingly contradicted by a eyetracking study [http://www.useit.com/alertbox/banner-blindness.html] from Jakob Nielsen. Their conclusion is that:
“Users almost never look at anything that looks like an advertisement, whether or not it’s actually an ad.”

That would suggest that blending into the site is better than having a high contrasting color. It is true that the first study was referring to recall and not what people look at. But if they don’t look at the ad that is going to reduce the chance of recall.

Sample ads Google uses for examples. Note that none of them would blend into a typical website.

So what are out recommendations? Test. Your circumstances can be different than a particular study or a general trend. We recently tested ads which blend in against ads which have more contrast. Surprisingly, the ads which had more contrast did much better with clickthroughs.