You can use Plaster of Paris, but you need to take care when applying coloration. PofP absorbs dyes very readily and can get too dark before you know it. Hyrocal, on the other hand, accepts dyes, but in more subtle shades since it's denser material. One way to keep PofP from getting too dark when dyed is to seal it with something like spray matte clear fixative. I'd also strongly recommend pre-tinting PofP with something like a pale shade of grey to kill the bright white; otherwise, when the castings get nicked, they expose brilliant white specks.
