The bitmap display (where the gui is drawn) is paletted, 8 bit. The palette varies between cameras, and also between playback and shooting mode.
Changing the palette shouldn't be too hard, but CHDK does not have this functionality, and it would require code for every camera, unless there's an eventproc that does it.
Look promising. May need to be registered with UI_RegistDebugEventPro and/or RegisterDialogAPIForEventProcedure