I tell ya that the gold fish farming is very long and tiring because of that, you can sell only the coloured ones so you fish the pond, sort the fish out on a wet table and put the brown ones back in the pond, grade the coloured ones and sell them. Wait a while and start again and again and again... At the end you get so pissed off by this mess that you just throw whats left in a predator pond an let them get red of fear
If you find how to make them change colour, you've found a gold mine as they are the most sold fish in the world and the first cost of the farming is to sort them out all day by workers as it can't be done by machinery. as long as they are brown or greenish they can change colour if they are silvery they won't change.
It has been noticed that it can more or less stimulated by green water.