Is the color of a dental cavity always black or brown?

Cavity: I am not sure if you are speaking of the cavity or the filling. Cavities are often invisible to the naked eye but can be seen on x-rays. If there is a hole in the tooth the cavity is probably quite advanced. Fillings on the other hand are either tooth-colored , silver or gold. Silver fillings can tarnish over time an appear black in color.

Dental cavities: Decay of tooth enamel starts as a dull white spot as calcium is etched out of the enamel. Advanced cavities that have penetrated into the dentin beneath the enamel can appear dark. But most cavities are not apparent to the naked eye until advanced, and may be more readily detected on x-ray. Please don't try to self-diagnose. See a Dentist w thousands of hours of training & years of experience.