Thank you for the information.
Under normal circumstances I would have suspected that this is just a normal color change as goldfish do indeed change color and usually to more of a drab/plain coloring but given the fact that 2 fish at the exact same time are changing color and due to the fact that there's been a chronic illness problem in the tank I would be inclined to believe that the color loss is due to unfavorable water conditions.
Supporting my theory is the fact that the tank is overcrowded and an overcrowded tank is quite susceptible to quickly deteriorating water quality.
My recommendations are simple:
Start off by changing 50% of the water in the tank making sure to use dechlorinated water that is the same temperature as the water already in the tank.
Add 2 teaspoons of aquarium salt to the water right after the partial water change.
Add a product to the water called Stress Coat which can be found at most major pet stores. Follow the directions on the product label for usage/dosage instructions.
Drastically cut down on the amount of food being fed to the fish. Feed only a small pinch of food once a day and no more. Excess feeding leads to excess fish waste which in turn deteriorates the water quality.
Ok---as a regular maintainance routine continue to change 50% of the water every 4-5 days making sure to continue with the minimal feedings.
Verify that the water temperature is in the proper range for goldfish which is 62f-72f degrees. Goldfish are coldwater fish and do not do well in warm water.
I'm not unneccesarily going to recommend a medicine because this is not a disease issue but a water quality issue and if you ask a pet store employee they will ofcourse say this is a disease issue so that they can sell you a medicine as this is how many pet stores make money.
Best wishes and please let me know if you have any questions.