Colour change is never normally due to lighting, only with diet. From experience, lighting has no effect on fish colour what so ever. If you use cheap wheat-based pellets then the colour will fade quite quickly over time. Flakes can be a bit to a lot better depending on the ingredients. The best foods for enhancing colour are krill, certain algae and aquatic plants. If you can't find any of them then go for a colour enhancing pellet.

Yep put in lots of greens and try color enchancing flake foods. Also I'd give them a bigger home too, a large pond would be ideal... How big are they?.

sorry but i must disagree. color enhancing food is just that, color enhancing. just as humans will turn orange if they eat a ton of carrots so goldfish will deposit plant pigments in their dermis and scales as well. but just as you can't truely tan by eating carrots, neither can a fish. the bulk of the true color comes from pigments the fish makes, not eats. the gold is from a pigment called melanin. if that does not sound familiar, it is the same pigment that makes humans tan in the sun. goldfish "tan" in UV too. the brighter the light the more pigment and brighter the colors. diet will help of course, but a goldfish kept in the dark will still fade despite the the quality of the food. a fish kept in a pond outdoor will have much darker colors than one kept indoors regardless of the food. chlorine can also bleach out the colors over time.

i breed goldfish and keep fish both indoors and out. the difference is striking. and to be honest i have not noticed any difference between quality foods claiming to enhance color and equal quality foods that don't. my indoor fish are vibrant orange, but turn fire engine red when moved outdoors. currently my outdoor fish are darker even though they have not been feed anything since october and what they get feed during the summer is relative crap compared to what the more prized indoor fish get fed.

white fish will get a yellow cast especially in the head (lemonhead) when kept outdoors, but this fades back to white in a few months when brought inside. i did check my food and you guessed it, what i feed the fish indoors has "color enhancers". the orange fish brought indoors faded as well. so that does not speak too well for the food over sun theory.

i don't discount healthy diet and water quality. a stressed or sick fish will lose color and look pale. i think color enhancing claims are marketing messages as much as anything. good food is what you should looks for, not claims about color.

i don't think it is worth installing a lighting system designed for a reef tank on goldfish tank. just my opinion, but good lighting will help. if you can grow plants, you have good light. buy a bulb from the fish store and tell them it is for growing plants. you might mention the fish fading too.

also goldfish do change color a lot. the black may not ever come back as it is fleeting in many breeds with age. i have had 2-3 year old solid black ranchu turn orange over a span of a month (outside). and seen solid orange almost red fish develop white spots long after their color should have been set. so don't expect a white fish to turn red as the white is usually the last color manifested. but if your orange/red fish is now salmon pink, i would put money on him turning back to orange when he goes back outside. we all look a little pasty after a winter under a flouresenct sun;)