Hi benbilly i can answer that one. As you correctly pointed out it is due to metal salts in particular Copper salts. What is happening is that copper (ii) ions present in water are being oxidised by reacting with Hydroxide ions in whatever products you are using. This oxidises them to Copper Hydroxide (Cu(OH2)) which has a green colour. It can occur if the products you are using are fairly basic, or if the water’s PH balance is off. It also occurs over time on the surface of copper in air, which is why some statues turn green.