Once when we got my first fish tank to be in charge of it, a family member notices something strange. While we were washing the equipments and the rockes Their clours went out ( the black rocks stared to wash out, the black stared to turn into white)!! it means they were colored. I guess strange colours are signs for unrealastic colours! ( black, red or blue etc...)
Since the coloures can be washed easily by water, we thougt it is dangerouse for the fish to have coloured rocks in thier tank. I did not put any coloured rocks except beige rockes, because the the colour did not go away when I wash them with water.