The seat tube is in fact 31.8...but your eyes are lying (and the picture quality doesn't help). It's all one color, very dark blue. They have done a black to blue fade in the past. Here's a link from a road test at Pez cycling. Unfortunately, the info is old. The test bike is one of the older frames when they were being built in Russia. But it does show a black to blue fade (that I wasn't wild about):

http://www.pezcyclingnews.com/?pg=fullstory&id=1502