Feature request: set over and under colours for colour map to top and bottom colours of custom colour map

[gdevenyi]

If I load a custom colour map where "white" isn't the top colour, particularly when using the 3D object display, its very inconvenient to find/copy/paste the RGB value of the over colour, it would be helpful if that were set when loading a colourmap based on the top/bottom colours of the map.