This command is used to manipulate the WM_COLORMAP_WINDOWS property, which provides information to the window managers about windows that have private colormaps. If windowList isn't specified, the command returns a list whose elements are the names of the windows in the WM_COLORMAP_WINDOWS property. If windowList is specified, it consists of a list of window path names; the command overwrites the WM_COLORMAP_WINDOWS property with the given windows and returns an empty string. The WM_COLORMAP_WINDOWS property should normally contain a list of the internal windows within window whose colormaps differ from their parents. The order of the windows in the property indicates a priority order: the window manager will attempt to install as many colormaps as possible from the head of this list when window gets the colormap focus. If window is not included among the windows in windowList, Tk implicitly adds it at the end of the WM_COLORMAP_WINDOWS property, so that its colormap is lowest in priority. If wm colormapwindows is not invoked, Tk will automatically set the property for each top-level window to all the internal windows whose colormaps differ from their parents, followed by the top-level itself; the order of the internal windows is undefined. See the ICCCM documentation for more information on the WM_COLORMAP_WINDOWS property.
Do not set this yourself unless you know exactly what you are doing.