|Warning: Colormaps are partially ignored by ZDoom because they are not compatible with many of the supported image formats. Instead, ZDoom offers true color blending, allowing to specify any powerup color, light color, and light fade with full support for all 256 possible light levels. The trade-off is the loss of compatibility with certain effects achieved through colormap hacks, such as replacing the inverse map of the invulnerability powerup with a blue map.|
COLORMAP contains 34 maps that map colors down in brightness within a palette. Its main purpose is to allow Doom to show sectors with different light levels.
The first 32 maps (0-31) are used for sector brightness. Divide the brightness by 8, rounding down, to get the colormap used. For example, a sector brightness of 7 uses map 0 while a brightness of 8 uses map 1. ZDoom does use this part.
Colormap 32 is used when the player has the invulnerability powerup. This colormap replaces sector brightness. ZDoom does not use this colormap at all.
Colormap 33 is left over from the Doom Press release beta and is unused in release versions. The beta actually used colormap 33 for invulnerability and 32 for the light amplification powerup. ZDoom does not use this part either.
This information is from the Unofficial Doom Specs.