I would have bet in a mild tone of blue (similar to the same App Store icon) would be one of the most common. Somehow, every time I swipe in multitasking most icons strike to me as blue-ish (Skype, Dropbox, App Store, Mail, Safari, Alien Blue, Tweetglass, Kindle, Textastic...) Probably just a too small sample, but a very interesting data check.