I have two identical hypostomus roseopuntactus that I got at the same time. One was placed in a 240 and the other one in a 40. The one in the 60 did the same thing as yours, but not as uniform. Some places didn't turn yellow/creamish. When the 40 leaked, I put it in the 240 with the other one. It turned back to normal within a month, but it's never the same, the spots and the base color just is not as good looking as the other one that remained in the 240.
I don't want to say that it's the water quality, because I maintain the 40 pretty well too, and all other fish in that tank was doing good. But in this case, the environment definitely made a difference.
