glutCopyColormap — copies the logical colormap for the layer in use from a specified window to the current window.
glutCopyColormap copies (lazily if possible to promote sharing) the logical colormap from a specified window to the current window's layer in use. The copy will be from the normal plane to the normal plane; or from the overlay to the overlay (never across different layers). Once a colormap has been copied, avoid setting cells in the colormap with glutSetColor since that will force an actual copy of the colormap if it was previously copied by reference. glutCopyColormap should only be called when both the current window and the win window are color index windows.
Here is an example of how to create two color index GLUT windows with their colormaps loaded identically and so that the windows are likely to share the same colormap:
int win1, win2;
glutInitDisplayMode(GLUT_INDEX);
win1 = glutCreateWindow("first color index win");
glutSetColor(0, 0.0, 0.0, 0.0); /* black */
glutSetColor(1, 0.5, 0.5, 0.5); /* gray */
glutSetColor(2, 1.0, 1.0, 1.0); /* black */
glutSetColor(3, 1.0, 0.0, 0.0); /* red */
win2 = glutCreateWindow("second color index win");
glutCopyColormap(win1);