The background or predominant color of your pocket square should not be the same as your shirt. Recent posted examples: A pocket square is just another layer of your outfit--on top of your shirt, tie and jacket. Moreover, it is the most superfluous of them all. So, if you're going to include it, it really needs to be sufficiently distinct. If you can't come up with a happy balance between matching your shirt (or tie or jacket) and incohesive randomness, just don't wear one. The lack of a pocket square has never ruined an outfit.