I wrote a couple of scripts as the request of a user at the gimp plugin registry to aid him in coloring lineart.

The one script (multifill) takes every area of the specified colour and fills it with a random colour, or a colour from a selected palette.

The second script (flatten, or an option on the multifill script) causes the colour to grow out, eliminating the drawing strokes.

The idea is that it gives different colored areas that can easily be selected (using the magic wand tool) for coloring, gradients, filling, etc on a layer under the lineart.

I was paying with Ascensions city script, and realized it could be used to randomly fill roofs/buildings with colours from a palette, for example, there is a palette called "bears" in gimp that is made of mostly browns.

Here is the source image:
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and after running multifill with the bears palette:
Click image for larger version. 

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ID:	10289

Now with a better palette, you could create nice shades of roofs/buildings....Using random colours then desaturating you would get a nice random heightfield...

(I warn you, it is slow if you have many small areas.....I think I should port it to python...)

You can download the script here:
http://registry.gimp.org/node/14051

-Rob A>