Since you had specifically asked for websites on your subject of interest, I decided to track a few down for you rather than composing my own answer to your question. I hope you find the following information useful.

According to:

fishdoc

It states:
If you leave your goldfish in the dark, you probably have noticed that it becomes a little paler at night. And you should also know that if your Goldfish is left indoors without full spectrum lighting, it will change from a reddish orange to a pale orange color with silver washouts.

The goldfish should not turn completely white for lack of light.

The orange color in its skin is not a "tan" - but the question is a good one, which brings me to a few points.

Color in the skin is enhanced by sunlight or other sources of natural UVA and UVB.

Pigments in the diet, carotenoids to be specific have a lot to do with the maintenance of color. There are a lot of carotenoids in shrimp and fish meal and there are color pigments in spirulina and haxanthin, a common additive in food.

Fish which change color, for example Red Caps which lose the red cap, or Blackand Gold or andas which wash out to yellow, or orange fish which eventually turn white are simply following a weak genotype.

The coloration and patterns of a Koi are in many cases, the thing that attracts people into the hobby of Koi keeping. These same two factors are also very important in determining the quality and therefore value of any particular fish.

Yet our understanding of fish and, particularly Koi, coloration is still an inexact science, which is plagued by theories, old wives' tales and relatively few facts.

My aim here is to provide an overview of Koi coloration and in doing so help to explain some of the mysterious changes you may have noticed in your own fish.

What makes color? The coloration of a Koi is produced by three color pigments which are contained within cells called chromatophores. The three pigments are Erythrin (red), Melanin (black) and Xanthin (yellow), each of which occurs, in different chromataphores. Complementing the color pigment are irridocytes which can be best described as tiny reflective spheres within the skin. All of the colors we see on our Koi are a mixture of these components. For example orange is a combination of red and yellow chromataphores; brown is a mixture of black and yellow and red is just the red chromataphores. If there are no chromataphores present the Koi will appear white due to the irridocytes. However, the position of the irridocytes within the skin affects its reflective properties. If they are on the surface of the scales the Koi will have a silvery appearance. If they are in the lower layers of the skin the fish will have a mat color.

In certain cases, the irridocytes can combine with the chromatophores to produce reflective colors (e.g. gold on the surface). Blue is an unusual color in that it is a result of deep lying black pigment with irridocytes in the middle of layers of the skin. The irridocytes interfere with the light to give a blue color.

Destiny of color. The chromatophores may be positioned on the surface of the skin (above the scales), immediately under the scales or deep in the skin. If the chromataphores are very dense the coloration will also appear dense, with the chromataphores on the surface of the skin blocking those below. However, the position of the chromataphores affects the 'stability' of the color. The chromataphores on the surface of the skin will often produce unstable coloration due to them being removed or spreading as the fish ages. Those deep in the skin are more stable and less likely to break up. The ideal is to have the same, dense color pigment in all layers of the skin. This results in both a dense and stable color.

Where does the color come from? Koi cannot synthesize their own color pigment therefore they have to consume it. In wild conditions the color pigments would originate from eating algae, shrimps, snails etc. In the confines of a Koi pond there is insufficient of these different organisms to satisfy the Kai's requirements, therefore it is important to feed color-enhancing foods. As with all Koi feeds, it is important that the color enhancing food given is of high quality to ensure that the pigments are in a form that the fish can absorb into its body. If color foods are not given to your Koi, the chromatophores would not be filled with pigment and the Koi will look pale or poorly colored. This can result in a Koi of high potential quality only looking mediocre. Feeding a color food would greatly enhance the appearance of such a Koi - but could not make a poor Koi great.