The dark brown color of a penny is most likely oxidation, which occurs when the copper in the penny comes in contact with oxygen. Since the average penny circulates for 25 years, a gradual accumulation of dirt and discoloration are common.
When copper is exposed to oxygen contained in moisture, air or a heavily polluted environment, it oxidizes and forms copper oxide. Over time, this changes a penny's color to dark brown or black. If left to oxidize long enough, the copper oxide turns green. A penny can be cleaned with a piece of cheesecloth and a solution of baking soda and water. The original finish cannot be restored.