My friend and colleague (I like him even more since he told me about this!) Segev gave me this information, which he had gleaned from some old posts on ROOTtalk and a lot of experimentation. It’s a technique for setting the color palette of ROOT plots so that, for example, Graph2D with the “COLZ” option doesn’t look like total crap. I have no idea why something like this isn’t used as the default palette in ROOT.
In short, here’s before and after screenshots of the plot.