ppmquantall(1) on Linux
The text d74ae517f2cf, as Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie), Debian 7 (Wheezy), Debian 6.0 (Squeeze), Debian 5.0 (Lenny), Debian 4.0 (Etch), Debian 3.1 (Sarge) shipped it.
run ppmquant on a bunch of files all at once, so they share a common colormap