Plaster rocks accept washes/stains very well. Foam rocks do not, and need painted. You can add color to the plaster when you mix it, or just do white, and wash/stain afterwards.
We prefer to pour plaster rocks white, put them in place, fill the gaps, and then wash/stain. You can do it either way.
Plaster rock mold:
Plaster rocks:
Foam rock molds in the canyon and across the front:
Plaster rock molds stained with india ink, and WS paint wash.
Foam rocks painted with artist acrylic and oil paint:
Plaster rock, primed with Gesso, and painted with artist paints:
We are in no means, rock experts, and all these were 1st timers. It's really a matter of trying different things on the bench to see which way you like best. Personally I like the white plaster rock with a wash/stain. In many areas of our layout we had to use foam rocks because of space limitations.