I have a Comet that has done the excat same thing and a "Chocolate Oranda" that was mainly a beautiful dark brown/black to the top and a shiny gold to the lower half. He is now bright Orange all over!! They just seem to change colour, some do, some don't. Neither of my Canary Yellows have changed colour. I have a Black Common Goldfish that had similar Marks, but it was getting too much attention from an over amorous goldfish but within a short while those white scales disappeared, got darker and are now definately black again.
Don't reckon it's ill tbh.