When you cast with foam (I've only messed with Smoothon's Foam IT! line) you can just pigment it black outright and brush in their line of metallic dust to give it that "shine" of metal. Painting flexible stuff is difficult since paint normally cracks when stuff bends.
Are you planning on making a mother mold for the silicon mold? The rule of thumb is that casting rigid things gets a flexible mold and casting flexible things gets a rigid mold.
For my rubber casting I use fiberglass just to save a couple of extra steps.