I am a fan of
potrace, and I recently read the Potrace FAQ.
The FAQ includes this beautiful one-liner:
cat img.gif | giftopnm | ppmcolormask #641b1b | potrace
The command I am looking for would return the codes of all the colors to be parsed by
ppmcolormask. It could return, according to
You can specify color five ways: o An X11-style color name (e.g. black). o An X11-style hexadecimal specifier: rgb:r/g/b, where r g and b are each 1- to 4-digit hexadecimal numbers. o An X11-style decimal specifier: rgbi:r/g/b, where r g and b are floating point numbers between 0 and 1. o For backwards compatibility, an old-X11-style hexadecimal num‐ ber: #rgb, #rrggbb, #rrrgggbbb, or #rrrrggggbbbb. o For backwards compatibility, a triplet of numbers separated by commas: r,g,b, where r g and b are floating point numbers between 0 and 1. (This style was added before MIT came up with the similar rgbi style.)