Pennies And Their Color

Copper turns colors depending on what they are exposed to. The normal changing of what we think of as a copper color turning brown is due to Oxygen. Many metal combinations utilizing Copper will turn darker from such normal reactions with Oxygen to form Cu2O, Copper Oxide. Note it takes 2 parts of Copper with one part Oxygen so this combination is normal due to the amount of Oxygen in the atmosphere. Brass, Bronze, etc will also darken due to the amount of Copper in that mixture. Copper when exposed to other elements will also combine to form compounds of different colors. Green is one of the most noticable colors found in substances containing Copper. This is also common and usually confused with silly terms such as Verdigris or PVC attacks. In reality once Copper is exposed to air, Oxygen, to form the Cu2O it then is now suseptable to combining with Carbon Dioxide and moisture in the air, CO2 + H OH. The resulting substance is a greenish patina called Copper Carbonate, [CuCO3-Cu(OH)2].
In reality all metals are attacked by substances like Oxygen and the resulting changes are color changes as well. Many have noticed the darkening of Silver when exposed to the air and spend many hours cleaning such dinerware. The Silver Oxide is very dark and unatractive for sure. Copper when exposed to H2SO4, Sulfuric Acid, will turn very dark green and if left alone will eventually all form crystals of bright green Copper Sulfate.
Crystaline structures containing contaminates in some instances are very desirable and the colors of Diamonds are an example.