The link does not lead to a picture.
What size is the tank? what kind of filter? Results of tests for ammonia, nitrite and nitrate?
When you changed the filter it uncycled your tank, your fish is struggling through a fish in cycle, this explains the behavior but not the color. It does not sound like ammonia burns. It is possible he is just changing colors, goldfish are know to do that, but I wish the picture had opened.
You can add Seachums prime as soon as possible. You need to do 25-35% water changes every 3-4 days until the tank is cycled zero ammonia, zero nitrite, but some nitrates. add add prime to all new water.