See Acidic Water, Iron, Manganese and pH
Staining usually occurs on cisterns, basins, baths and piping and can be due to low levls of pH in combination with Coppr, Iron, Manganese and other salts .
Green usually designates pH or Copper
Red for Iron
Black for Manganese
These are just a rough guideline. Electrical earthing of geysers to the water mains may well cause a current or elctrica filed which can cause the piping to corrode.
It is imperative that you filter your water to a quality that you can rely on and in a way that when municipal systems fail you will still be safe