PLAYPAL

A PLAYPAL is really a collection of palettes used for various purposes. It has a total of fourteen palettes, each 768 bytes. Each palette has 256 colors of three unsigned bytes each (0-255); one for each of red, green and blue. Furthermore, the COLORMAP resource also affects the display of colors on screen.

The palettes each have a specific function:

An examination of the Doom source code reveals that the unused palettes (1 and 9) were likely intended to be the first levels of the red and white tinting effects. Because of the logic used in the palette code, they are never used.

This information is from the Unofficial Doom Specs.