Pigment in goldfish is found in pigment-producing cells called chromatophores, including xanthophores (containing yellow pigment) and erythrophores (containing red pigment). When fish are outdoors, they will receive ultraviolet light (including UVA and UVB light) from the sun. UVA and UVB are both high-energy light, and can be harmful to organisms, however, pigments can provide some protection from this, and ultraviolet light stimulates the chromatophores to produce more pigment. Artificial lights usually only provide a limited spectrum of light compared to sunlight and usually do not produce ultraviolet light. This means that over time, goldfish chromatophores exposed to artificial light will produce less pigment and the goldfish will look paler in color.

The UV light not only impacts the amount of pigments produced, but also the density of the pigments or how spread out in the cell they are, which also affects the coloration of the fish. This response is pretty rapid, so you might notice that when you turn on your fish tank light in the morning, the fish may look paler, but will look more vibrant later in the day.

The coloration of the fish is also due to which pigments are expressed in which combinations, so the type of artificial light may impact the balance of the pigment color types and therefore the overall color of the fish over time. The food the goldfish eats will also affect its color; most goldfish diets contain natural pigments called carotenoids and the amount and type the fish eats will impact the color of the chromatophores.

The longer a fish is away from natural sunlight, and the type of artificial light it receives, as well as the type of food it eats, will all impact the degree the color of the fish will change, and how long this process will take. As the chromatophores die, if they are not replaced by other similar chromatophores, the color of the fish will change. These effects can be observed in some fish in less than 30 days.