You might want to check out the examples that come with the OpenCV library - it supports identifying regions (called blobs) of similar color.  Might be a good starting point for something like this...

I think the kind of sketch whose result you show is done with the growing rectangles algorithm: the idea is to pick a number of random places, then to make rectangles grow from these points, picking the color of the selected pixel. They grow until they hit another box, or perhaps up to a given size. When they stop growing, you pick new points, not covered by these boxes, and you iterate again.

Looking closer, the algorithm is described in http://complexification.net/gallery/machines/boxFitting/