Hey Dragkon here again.
When I say simple I really do mean it. The first stage is to paint the building in Dawnstone, you don't really need to under coat it in anything as you are just painting it a slightly darker gray than the plastic.
The next step after that is a dry brushes across the whole thing with Ulthuan grey, to brings out almost everything it on it.
Once that was done I went along adding silver to parts of it just to give the building a bit more detail, any skull wall was painted with rakarth flesh with a Nuln oil wash applied over those parts leaving the stone effect be.
So a couple of buildings done this way and hopefully get some more stuff done up in this style.