A cast has two layers: a soft layer of padding to protect the skin and a hard outer layer that keeps the broken bone from moving.The outer layer is made of plaster or fiberglass. Plaster casts are made of Plaster of Paris, a white powder. Plaster casts are heavier than fiberglass. It's easier to write on them! Fiberglass casts are lighter than plaster casts. They come in an array of colors. Purple and hot pink are girls' favorite colors. Boys like blue; green camo, too.       –Parker Medical Associates, a nationwide supplier of fiberglass casting materials Plaster and fiberglass come in rolls or strips. The doctor or technician wets the material and wraps it around the soft padding that's already on the broken body part. The material gets hard quickly.