Poor water quality is the most common cause of color change in goldfish.

You mention 50% water changes. If you really want to keep these goldfish, you probably need to step up to 80% water changes.

Goldfish are big polluters. In a 20gal tank, they are likely too much bio-load for that tiny bubble filter you have (unless there’s another filter I’m missing) to keep up with, and all of the fish are going to suffer.

Remember: the nitrifying bacteria that reduce harmful ammonia and nitrite to relatively less harmful nitrate only live on the surfaces of substrate in your aquarium. That means that your filtering capacity is limited by how much water passes through it, and your little filter is low-flow and small.

Unless you can add significantly more filter capacity (underground filter that pulls water through the gravel, or an external filter that pumps water through cartridges or high surface area media – which still may not keep up with these dirty fish), my recommendation would be to remove the goldfish.

All that aside, goldfish can also change color when they are growing very quickly. They will eat as much as you give them, and if they get more than they need, they will grow very quickly. There used to be a myth that they grow to the size of their environment, but I’ve seen them in a 55gal and remain one inch when fed sparingly, and also have seen them grow to six inches in a 10gal.

Goldfish are good starter fish because they jump-start the ammonia cycle and they’re relatively hardy. I don’t like them as long term community members because they’re dirty and can increase the workload for new aquarists, causing stress for other fishes and discouraging beginners from the hobby.

Edit: typos