Pseudocolor

The pixel value in the frame buffer selects an entry in a colour table which contains the intensities of the three primary colours (Red, Green, Blue). This is how the traditional image display from the days before X worked and there are typically 8 planes in the frame buffer and therefore 256 entries in the colour table. Each colour table entry typically has 8 bits for each component giving 256 levels of red, 256 of green etc. Put another way, at any one time you can have up to 256 different colours out of a palette of 10$^{24}$ ($\sim$16 million) possible colours.