Things like statues, pennies, or pipes made of Copper turn green over a period of time. This happens due to oxidation. Oxidation is the process in which the electrons are removed from the substance. To be more precise, copper turns green due to copper carbonate, found on the top of its surface.
When a metal comes in contact with moisture, it results in oxidation and copper changes its colour. A famous example is that of the statue of Liberty which now has a green hue but old timers may still remember when copper was clearly visible. The same happens to the copper pipes and tanks which acquire a green hue with the passage of time.