pnmcolormap

Create quantization color map for a PNM image. More information: https://netpbm.sourceforge.net/doc/pnmcolormap.html.

pnmcolormap n_colors path/to/input.pnm > path/to/output.ppm

pnmcolormap -splitspread n_colors path/to/input.pnm > path/to/output.ppm

pnmcolormap -sort path/to/input.pnm > path/to/output.ppm