I too used the rigid foam but in blue as our dyi store don,t carry the big sheets. I use the talus and this is what I did.
I used and old blend or to get some smaller pieces so I can vary in size. I then sprayed with soapy water on a piece of card board. at his helps break the surface tension which you will need to do when you paint the carved rocks.
After the soapy water dried I used various Michaels acrylic paints to meet my desired color. In smaller amounts I used a mixture of acrylic paint added to water and dish detergent to create a wash.
I used the same process on my carved mountains and rocks shown in picture.
Let dry between coats adding other colors as desired in the same formula. Hope this helps
