Genetics:
Some goldfish breeds have genes that cause them to change color as they grow. For example, the popular "calico" goldfish starts life as a dark brown or black fish and gradually develops orange and white spots as it matures.
Environment:
The environment in which a goldfish lives can also affect its color. For example, goldfish that live in dark tanks or aquariums may become paler, while goldfish that live in bright tanks or aquariums may become more vibrant.
Diet:
A goldfish's diet can also affect its color. For example, goldfish that eat foods that are high in carotenoids (such as shrimp pellets or freeze-dried brine shrimp) may develop a more orange or red coloration.
Stress:
Stress can also cause goldfish to change color. For example, goldfish that are exposed to high levels of stress (such as being chased by other fish or being kept in a small tank) may become darker or develop pale patches.
Disease:
Some diseases can also cause goldfish to change color. For example, goldfish with bacterial infections may develop red or white patches on their skin, and goldfish with fungal infections may develop fuzzy patches on their skin.
If you notice that your goldfish has changed color, it is important to observe it closely to see if there are any other changes in its behavior or health. If you are concerned about your goldfish's color change, you should consult with a veterinarian.