Color/darkness represents the number of ZIP codes within each pixel.
I'm using the "CubeHelix" color scheme (shameless promotion: I wrote the first version for IDL). This scheme is great because it de-saturates from color to black/white while correctly preserving brightness. It can be rainbow colors too, not just monotone like above.
For example, here is the same figure, but with the color turned off (using Apple's Preview app). Great for making figures that look good both online (where color printing is free) and in black/white print (e.g. ApJ charges $350 per color figure, last I checked)