Really surprised no one has yet posted the software that’s almost certainly used to create these images, the Measurement Factory’s ipv4-heatmap package, on GitHub here: https://github.com/measurement-factory/ipv4-heatmap

And, yes, it was absolutely inspired by Randall Munroe’s xkcd Map of the Internet, which someone has already linked.

There’s a fork that uses ColorBrewer palettes, which may be what they actually used: https://github.com/hrbrmstr/ipv4-heatmap

(I used to work at Akamai and used this to produce monthly versions with each /24 colored based upon how many addresses within it had connected to the company’s network.)