Being in California, I tried a couple of the available materials, and I ended up using stuff.Easily available through my local hobby shop.
I use their HS2 and HS3 silicone rubber.
I use their white resin with a dab of black coloring dye to make it light grey.
I add the coloring to the "A" bottle before I start casting.
That's it.
Later I'll get into the differences of materials