I've been looking at Ascension's Planet tutorial (again ) and I'm trying to replicate the land colouration of his Earthlike planet in Gimp.

Without success.

With a few clicks of the mouse, using some form of gradient overlay, Ascension takes a greyscale cloud on his land area and converts it to a coloured terrain that merges from white ice at the poles, through a green temperate zone, to a yellow desert at the equator.

Can anyone give me step-by-step instructions on how to do this in Gimp, cos it's got me beat?