Green board is a type of drywall used in the construction of commonly damp areas of the home -- such as the bathroom, kitchen, and laundry room. It's face and core are water-resistant, but not waterproof. Sometimes contractors will use green board when building a shower or tub surround to save cash, but this is not recommended. Doing so will inevitably lead to mold growth and other potential hazards. When constructing a shower or tub surround, use a cement-based backerboard instead. Why the green color? It's simply for contractors to tell the difference between green board and regular drywall.