First of all, exposure to sunlight or full spectrum lighting will help the fish develop its colors. The skin cells actually contain pigments that define the red and yellow appearance seen on your pets. For example, like goldfish and koi, they actually have buildup of carotenoid which is derived from the food that they eat. With exposure of sunlight, this will help the cells stay healthy and these pigments will develop further and thus projects the beautiful coloration that you see.
As what I always stress, try to avoid placing the aquarium on a location whereby it is subjected to longs hours of sunlight (to avoid algae problem) but somehow a short period of exposure should be fine. For me, this is more than good enough because what you will notice is that your fish’s natural color will get stronger and if you are staying at a place whereby you can’t find the right location to get the brief sunlight exposure or you just decided not to rely on sunlight, then try to get aquarium lighting which provide full spectrum of light.
As mentioned earlier, diet and what you feed to your fish is also equally important. Depending on what you provide your pets, certain types of high quality fish food sold in aquarium stores are mixed with Astaxanthin which is the main ingredient derived from prawn shells that contain the essential compound that will enhance the fish’s natural colors. Apart from that, do not rely solely on processed foods for your pets because sometimes you need to mix in the live foods as well to create the balanced meal and I would recommend you use a mixed supplement of bloodworms as natural feed combined together with high quality fish pellets. If these are provided in the right amount, it will definitely help to bring out the colors of your fish.
Water and living condition are also other two areas which you need to look into. Make sure that the water is clean and do remember that crystal clear water might not indicate that ammonia is not present. Thus make sure that you have your test kits in hand to conduct a simple analysis to confirm water quality is fine. Temperature is also something you need to take care of as well. Higher temperature does not necessarily means it is fine for your pets as different species of fish has different range of temperature level that they prefer.
Somehow at times, introduction of live plants combined with use of backdrops and sands will also bring out the color of the fish. Well although you can argue that it is not something introduced or ingested by the fish, a simple explanation is that change of surrounding and environment can actually affects the mood of your pets making them happy. Do not rule out this possibility because it does make sense.
Other reading materials:
Factors that can cause fish to lose colors