In the case of iron or alloyed steel the iron oxide is similar in color to iron and is hardly noticeable. With copper there is a distinguished color change when oxides form. Nitric acid is a substance rich in oxygen and is used by patina craftsmen in patina solutions to deliver the oxidizing oxygen. Moisture increases oxidation by forming hydrates. When exposed to moisture, iron or alloyed steel forms the familiar orange-colored hydrates of iron oxide commonly called rust.
Copper turns greenish-blue when hydrates form.
Heat opens the pores of porous metals and allows oxidation to lock inside the metal. Heat also evaporates moisture from the metal facilitating oxidation. Metal that continually gets wet then dries oxidizes more quickly than metal submerged in water. The chemicals that produce these colors are made by combining a metallic element with nitric acid. The nitric acid is the oxidizer that attaches the element to the bronze and the element provides the resulting color.