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A PPM color map is a regular PPM image that is used by some programs to define a set of colors.
ppmcolors generates a one row PPM image that contains one pixel of each color representable by the maxval you choose. The maxval of the PPM image is that maxval. Note that you can change the maxval of the color map by running the output of ppmcolors through pnmdepth. As long as the new maxval is a multiple of the original, the resulting set of colors will be identical. If the new maxval is not a multiple, the resulting set of colors will be slightly different.
When you select a maxval of 5 (which is the default), you get a color map of the set of "web safe" colors as defined by Netscape. Most web browsers guarantee that they can produce at least these 216 colors (215 plus black).
pnmcolormap is another program to generate a color map. It chooses a set of colors designed to represent the colors in a specified image (or simply the set of all the colors in that image, if you choose).
pgmramp performs a similar function for PGM images.