[Marcello]

"-Undergrowth, shrubland, chapparal, call it what you like. The idea is something intermediary between grass and forest, made up of tangled man-height tree saplings that make passage very, very hard for both infantry and vehicles."

That sounds like a good idea.

"Just for the look of it, I would really like a lighter rocky terrain, maybe with some inbuilt vegetation."

There is already a rocky terrain with some built in vegetation: summer rough. Nothing whitish but as it has already been noted what are you looking for is actually a white impassable.