Red eyed tree frog eyes fading?

[ohyoufancy]

does anyone know any reason for the eyes in an adult red eye tree frog to fade? a friend of mine knows of a frog that was bought as an adult about 4 years ago and his eyes which were bright red have faded to a pinker color over time. wondering if its some kind of vitamin deficiency. some food for thought, the frog is otherwise perfectly healthy, still eating and retaining all other colors.