When ever you don't feed him or her.
goldfish do
Goldfish are not particularly attracted to any one colour.
the lack of light being in the dark
Yes!
It will eventually turn white.
Global warming
Actually, goldfish don't have a specific gender. They are born as females, but as they grow, they turn into males.
A goldfish left in the dark would gradually, over time, turn completely white. This is because the color pigmentation cells responsible for the fish's bright color, cannot work without sunlight. It would, however, take a very long time for a goldfish to lose 100% of its color because they have so many color pigmentation cells in their scales.
goldfish turn blue when they are asleep
If a goldfish is kept in the dark it will appear lighter in the morning, So if left in the dark for a long period of time, a goldfish will turn almost white. If kept in the light, it will absorb color, and often become a brighter gold. (almost the same as when we tan, the sun gives us color, and when we get less of it we turn whiter, or lighter)
It's just getting a new gold color
It will turn white, or at least will start having some loss of color.