Some people complain about their black and white goldfish losing their black pigment. This is common in goldfish and the process is called de-melanization process. Well, my late experiment with breeding black and white oranda from Thailand (not from the blue metallic scale, but from the grey metallic scale as I wrote in my last post) shows quite a strong character of melanin (black pigment). The black pigment can reappear or grow. This is different from stress fish which often shows some temporary black pigment which will be gone on several weeks (or months). I am tempted to call this phenomena as re-melanization process. I notice that this changing pattern also happens in koi world in the kumonryu variety.
Well, not all black and white oranda from Thailand shows this characteristics. It just happen that the line on my hand is very strong in this unique character, and it happens in my ponds at least five times (as I noticed), which is quite often compared to my previous twenty years of experience in the goldfish world. I think I should preserve this gene and hopefully introduce it to other goldfish varieties. Below shows two fishes, one had lost all its black pigment and then regained it in just one month, and the other had a growing black pigment in the same time period.
hermantogoldfish 
re-melanisation – Do you think it has got to do with the water quality (hardness, temperature, or something else.) . I somehow notice that tri-color has the tendency of getting the blacks back
I do not really know the answer. In the koi world, they says hard water will enhance the black color. In my pond, I don’t think the water is hard, since my fishes lay eggs simultaneously there. So, I am sorry I cannot answer your question.