Fill or ground water can cause copper, iron, and manganese can find their way into the pool. The copper and iron can also be from metals in accessories like the pump or heater that are corroding due to improper water chemistry.
If the pool’s pH, total alkalinity, or calcium hardness level is low, then the water tends to be corrosive and the “rusting” of metal parts in the pool is greatly increased. Another common source of copper is copper-based algaecides. The cheaper ones often supply copper ions rather than complexes and, if too much is used, will result in green pool water.
If the water changes color after chlorine (or any other oxidizer) is added, the swimming pool water probably contains a metal.