Side note: looking at the color choice in figure 3 (http://www.pnas.org/content/early/2015/10/29/1518393112.full...):

"Census regions are Northeast (blue), Midwest (red), South (black), and West (green)."

I think they were picked manually. On the one hand, I think picking that makes them more memorable, on the other hand, it surprises me that people in the politically hypercorrect USA would pick those colors this way.