HELP! My fish is changing color!

In goldfish, this is quite common. Goldfish have a genetically predetermined colour, but they also have genetically predetermined colour changes throughout life. Don’t be concerned if it changes between natural colours. (orange, white, red, gold, black).

In Bettas, AKA Siamese fighting fish , color changes can happen periodically. Do not be concerned unless it gets very dull and grey, a sign of stress that may be caused by disease, but usually occurs right after a water change, being moved, or another sudden change. If it seems to get a bit of short, rust-coloured fuzz on it, as this is a disease called Velvet. See https://www.petfish.net/kb/entry/12/171/

Angelfish will often intensify or completely eliminate thier stripes or markings. Strong markings mean they’re excited, stressed or afraid. Dull markings usually mean they’re relatively calm, from my observation.
In all other fish, change of colour is usually not good, especially if it gets dull. Look into you water quality and check for Signs of disease.