From my experience with the Parrot fish,(which is only recent) they are usually one color as juvies and as they mature the color will change. My guys were very dark brown with only orange on the fins and then it became orange with black splotches and now one of them is almost all orange with some light orange in places. The other younger one is still all dark with just orange on the fins and has not started to change color yet.
I cant say for sure, but I suspect your Parrot is just going through color changes and hopefully nothing more. But you may want to keep your eye on it for any behavior changes in eating and its interaction with your other fish to make sure that it is not sick.
EDIT:
I just read your aquarium info, it is up to date, and the Parrot fish are in that tank, I would have to say that it is very overstocked and without knowledge of the
nitrogen cycle I am wondering if perhaps you are having a water quality issue?
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