TOG-CUP
The Open Group's X11 extension for sharing the default colormap with colours somebody else had already claimed.
On a 256-colour display the default colormap was a scarce resource every program fought over. TOG-CUP, the Colormap Utilization Policy extension, arrived with X11R6.3 in 1996 and gave clients two things: a query listing which cells the server had already reserved, and a way to store colours at specific positions. The first was aimed at X servers running on top of Windows, where the host had already locked its static colours and an X program would otherwise allocate an almost-identical shade right next to them. The second suited CDE, whose desktop colours were supposed to stay put no matter which application started first.
Few programs bothered. Once 24-bit TrueColor became normal around the turn of the century there were no cells left to negotiate over, and the extension turned into a name in the xdpyinfo list that nobody could quite explain.