These materials are normally nonconductive, but can be made conductive by supplying the right energy. Inducing conduction in semiconductors is the whole basis of solid-state electronics. Some semiconductors have such narrow band gaps that any visible photon can bridge them. They therefore behave optically much like metals. They typically appear red or yellow as a consequence.
Sulfur is one such material.
Semiconductors with wide band gaps absorb no wavelengths in the visible light range and are colorless. Diamond is one such semiconductor. Impurities in diamond can introduce color. Atoms that introduce energy levels capable of accepting a valence electron are termed acceptors. Other impurity atoms have electrons capable of jumping to the conduction band.