The colors come from the current colormap. You can go all out and
make your own colormap and color based on index using your map. I
don't know how to do that off the top of my head though.
The colors may come from the colormap, but how do they get assigned to
areas of the plot itself? The most obvious way to do it is to index
by z, so areas with low z are shown with a cool color, and areas with
high z are shown with a hot color (assuming you are using a cold-hot
colormap).
As far as I can tell though, this is not the system currently used,
nor is there any coherent system used.