A ColorMap is a collection of 2 or more number of RGBA colors. They can be expressed either as a string of numbers, or pointing to an image file where each pixel will be used as a color.

Example:

This gives a colormap with 2 colors, first one blue and second one white:
colormap=1 0 0 0 1 1 1 0

Or it can be pointed to a bitmap file:
colormap=bitmap_file.tga

Colormaps usually describes how the colors of an entity will shift during its lifetime, where the first color will be used at the beginning and then cycle through all colors untill the last one is use at the end of its lifetime.