If a goldfish is left in the dark it will turn pale!
Goldfish follow the same basic principle of tannign with humans. In the same way that we produce pigmentation when exposed to light, goldfish have a stronger color when exposed to light. Natural light even makes their color stronger than artificial light.
If they're not exposed to enough light, they begin to lose their pigmentation and turn a few shades lighter, though not completely white because they also get some pigmentation from food. This is even noticeable if they're left in the dark at night!