PLAYPAL

From ZDoom Wiki

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

The palettes each have a specific function:

0: normal

1: unknown

2-8: progressively more red (8 is most red). Used to show pain when the player is hurt, and reddens the screen when the player picks up a berserk pack.

9: unknown

10-12: progressively more white. Used when the player picks up items.

13: radiation suit

This information is from the Unofficial Doom Specs.