Goldfish is turning black

I do not now if this belongs here or under disease.
I got 6 gold fish from my grandmas pond cus it froze over and killed 5 one pound koi
About a week ago it started to turn black it is a 13 cent feeder that is about 6 inches long orange and white and the other 5 r fine no color change at all what could it be
Any ideas would be great
Thanks

my pond up in Iowa used to freeze over every winter and my goldfish where fine now the pond was about 6 feet deep. how deep is your pond? and how big?

The pond is about 3 ft deep and not shore how many gals it is my grandmas
Well last winter it froze and she last all her fish so she did not want to lose them
Again so they are in my 90 doing fine in the spring we will reintroduce them and maybe add one or 2 koi she said that she does not want them to die is it the tank or maybe the fish with them or the temp dif

how deep does the ground freeze where you are at? because your pond need to be at least 1 foot deeper then that so that the fish can survive in the unfrozen part

First I would have to say that your grandmas pond is not deep enough. They should winter over just fine if it is.......As far as the goldfish turning black I wouldn't be to worried about that. Goldfish are quick change artists and can changes colors at any time. If it's acting normal don't worry.

I agree Whisper