glutSetColor sets the color of a colormap entry in the layer of use for the current window.
void glutSetColor(int cell, GLfloat red, GLfloat green, GLfloat blue);
Sets the cell color index colormap entry of the current window's logical colormap for the layer in use with the color specified by red, green, and blue. The layer in use of the current window should be a color index window. cell should be zero or greater and less than the total number of colormap entries for the window. If the layer in use's colormap was copied by reference, a glutSetColor call will force the duplication of the colormap. Do not attempt to set the color of an overlay's transparent index.