Polyester casting resins are a great and cost-effective choice for producing all kinds of artworks and models, typically from moulds. They can be either opaque or clear and can be coloured with addition of pigment paste, at a rate of about 1% - 5% - A little pigment goes a long way!
They are supplied as a 2 part system comprising 2 chemical liquids - resin and catalyst. When the catalyst is added to the resin (approx 2%) this causes an exothermic (giving off heat) reaction which "cures" the resin and makes it form into a solid plastic state.
For stronger casts that give off less odour you can look at polyurethane (PU) resins.or epoxies