I'm just guessing here, but it looks like they stamped the image in a grey colour and then watercoloured over it. They may have used gauche paint which is kinda like watercolour but has more opaque pigment in it and uses white (which true watercolour doesn't). Then it looks like they went in with some black paint, which again, true watercolour doesn't use. Some of the areas, like the mountains look like they used transparent washes, but the structure looks more like the opaque paint.
As I said, this is just a guess, maybe someone else has done this and knows more.