ppmtomap (1) - Linux Manuals
ppmtomap: extract all colors from a portable pixmap
ppmtomap - extract all colors from a portable pixmap
This program exists only for backward compatibility.
Use pnmcolormap, which replaced it in January 2002.
One trivial difference between ppmtomap and pnmcolormap all is that if the input is PBM or PGM, ppmtomap would produce PPM output, whereas pnmcolormap all produces the same kind of output as the input. This should not be very noticeable, though, as PBM and PGM images are usually usable anywhere a PPM image is.