Hi! Based on the large size of your goldfish and the small size of the tank, the unavoidable fact of the situation is that the goldfish will produce far more waste than a tank that size is capable of dealing with. The presence of 0.25 ppm
Ammonia is proof of this and is a high enough concentration to cause serious damage to the health of a fish in the long term. As far as the fish changing color is actually is pretty common with goldfish to go through such a color change at some point in their life so I wouldn't worry about that as long as he is acting normal (eating, pooping, swimming).
I don't know what the current water change schedule on the tank is, but I recommend increasing the frequency of water changes to help combat the water quality issue which will end up shortening the lifespan of your fish. What is your current maintenance schedule on the tank?