Well, got to make this quick...
So, Monkeyboy--sure, we can try any color of the rainbow. What did you have in mind?
So I was going to add a decoration along the top of the wall, and an inner edge to the door. And I agree with you about the size of the wall, Ruvinka; I don't want the wall to dominate the the building either. I'm thinking that it might be wrapped too tightly around the sides of the building right now. Well, it'll be easy enough to make it wider.
Monkeyboy, we can really do just about any type of geology we please; I think that white quartz boulders might look nice, or black-grey basalts. Have any suggestions for rock types, Ruvinka? The rocks in the big red-->green valley picture I did were metamorphic rocks with strong layering in them. Now Monkeyboy, from a realistic standpoint, limestone might be a problem since this area has a lot of rain (being a jungle); the limestone would get dissolved and then sinkholes would be created into which a building could collapse. I suppose, however, that as long as we don't build the ERC on a sinkhole, we would be alright.
Limestone does make a great building material. In addition it can be almost every normal shade of rock coloration too (reddish, brownish, greyish, blackish.)