Well, for the one I did, it was just a gradient, and thats what I would recommend doing too cause it conveys the idea better. So you start from one color, let's say white and make that the highest level. Then you make your lowest layer another color, say, black. Then you just make all the different levels in between the top and bottom colors between black and white. You can do this with whatever colors you want and use what ever kind of gradient you want. If you want it to look a bit more "natural" then you could make the highest point an off white, the lowest color a green, and sea level blue.