A lot of people say the typical blue title AdSense ads convert the best. I used red for two months straight, and it converted about 5% better than blue. And no, my site was not red theme.
Well it all depends on the niche and the layout of your site. Most of the time I used blue. Because a lot of old internet surfers associate blue as a link, so they pretty much aware when clicking it.
I have tested a few colors and come to a conclusion that different colors work better with certain sizes. I am currently testing a purple color on my square ads, but it's not converting as well as with purple tower ads. However, blue tower ads converts lower than blue square ads. I think it all depends on how your site is structured and where your users naturally focuses. You could either use a color that says "Hey look at me" or one that says "I'm a part of the website". If you get what I'm saying.
i use red and orange on some of my website, they are providing good ctr too Just need to ensure your ads title is obvious
click rate depends on your visitors and keywords. If you got good visitors, then you'll have high click rate or if you got dumb visitors, then nothing