I can only speak from personal experience with my own lay out and the train club I belong to. We use hydracal for our base because it offers a very strong yet light
base structure. You are right about trying to color it, it is quite hard and does not take paints easily. Myself and the club use
good old plaster of Paris for rock molds and
fill ins for hydracal. The "Sculptamold" is
was formulated to be used on styrofoam base
material. It is quite pricey. If you are starting out with a good sized layout, you should purchase a bag of it from a masonry supply outlet. I've only been able to purchase it in 100 lb. bags, you can sure use it up in a hurry though and if you don't keep it in a dry area and it will last for ever.
Bob Cochran
AKA BabyChoo@aol.com