Run
pnmquanton multiple files at once such that they share a common colormap. See also:pnmquant. More information: https://netpbm.sourceforge.net/doc/pnmquantall.html.
pnmquant on multiple files with the specified parameters, overwriting the original files:pnmquantall {{n_colors}} {{path/to/input1.pnm path/to/input2.pnm ...}}
pnmquantall {{[-e|-ext]}} {{extension}} {{n_colors}} {{path/to/input1.pnm path/to/input2.pnm ...}}