I've been doing this with my Raven Guard Space Marines. I used a grey colour scheme for the sand (black, drybrushed with codex grey then fortress grey), and GW's yellow Dead Grass.
The grey sand was supposed to represent an urban environment, crumbled concrete and other urban debris but I wouldn't recommend it, doesn't look very realistic. A neutral brown sand to represent soil, with more realistic urban debris (e.g. sculpted bricks, paving stones etc) and some grass scatter would be better.
For my Mines of Moria Fellowship, I'm sculpting (or rather, carving) cobblestone bases out of milliput, and painting them in a variety of colours to represent a cobblestone road in Moria. Adding some grass scatter would make it appear like an above ground city such as Osgiliath.
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Its a lot of work though as the tabs have to cut away and the models pinned via the legs to the base. I've only completed Boromir so far.
