OpenGL supports both RGBA and color index rendering. The RGBA mode is generally preferable to color index because more OpenGL rendering capabilities are available and color index mode requires the loading of colormap entries.

The GLUT color index state variables are used to read and write entries in a window's color index colormap. Every GLUT color index window has its own logical color index colormap. The size of a window's colormap can be determined by reading numColorMapEntries.

GLUT color index windows within a program can attempt to share colormap resources by copying a single color index colormap to multiple windows using copyColormap. If possible GLUT will attempt to share the actual colormap. While copying colormaps using copyColormap can potentially allow sharing of physical colormap resources, logically each window has its own colormap. So changing a copied colormap of a window will force the duplication of the colormap. For this reason, color index programs should generally load a single color index colormap, copy it to all color index windows within the program, and then not modify any colormap cells.

Use of multiple colormaps is likely to result in colormap installation problems where some windows are displayed with an incorrect colormap due to limitations on colormap resources.

Controls the color index colormap entry of the current window's logical colormap for the layer in use. The layer in use of the current window should be a color index window. The color index should be zero or greater and less than the total number of colormap entries for the window (see numColorMapEntries) and different from an overlay's transparent index (see transparentIndex).

If the layer in use's colormap was copied by reference, setting a colormap entry will force the duplication of the colormap.