What is Patina?
Patina is a thin layer that forms on the surface of stone, copper, bronze and similar metals through age and exposure. The Statue of Liberty has it. Old church roofs have it. That gross penny you found in the gutter? Yup, the mottled greenish blue color is patina.
Patina differs from paint in that it cannot be strictly controlled. It takes a talented artist to make patina finishes look natural and not forced.