Pure copper is unstable and oxidizes under atmospheric conditions. Zinc makes brass way less corrodible than copper but still different conditions of corrosion can occur ranging from a protective patina rich in mostly copper-oxide through to dezincification if the zinc content is too high. Exposed to normal moisture in the air, patinas form that are mostly greenish blue (normal oxidation/reaction with oxygen/air & water). Below you will see changes in appearance during weathering cycles for different alloys.