Forum Moderators: martinibuster
Lurking on this board for a while now, I've seen that question a lot. The answer is usually: experiment. For some people, ads that blend in work better. For others, ads that contrast work better. It depends on your site and audience.
Set up a ton of channels in AdSense and try a few out for a week or two and compare.
I have seen examples of ads that were blended in so well, that users might be surprised (if not upset) about the landing page. Whether or not that improves "site stickyness" (and whether this is something desirable) is something you can judge best.
In addition to what scyzoryk said, I would like to point out that ads that are clearly identifyable as ads might result in lower CTR, but the click quality might be better and thus result in higher EPC.
Also, if you have a site that users don't try to flee as quickly as they can, having blended-in ads can lead to "ad blindness" as users move from page to page and assume they've already seen the ads. Having ads in contrasting colors, using a rotating color pallette, conveys a subliminal message of "Hey! These ads are new--they aren't the same ads that you saw on the previous page."