Some articles on colormap:

X Window System Core Protocol - Colors
... affect the representation of colors the color depth the colormap, which is a table containing red, green, and blue intensity values the visual type, which specifies how the table is used to represent colors ... If the client can change the entries in the colormap, this representation is identified by the PseudoColor visual class ... class StaticColor is similar, but the client cannot change the entries in the colormap ...