That's a normal and temporary color change, nothing to be concerned about.
when i had mine, it would changes colors depending on mood. my current sailfin does the exact same thing. when i feed the tank, he is almost see thru... like i can see inside of him. its really wierd, but then others times he is super colorful and the best looking sailfin i have ever seen.
i have heard the color fading a bit as they get older, but i have never seen this first hand. none of mine have ever had this happen.
That's a normal and temporary color change, nothing to be concerned about.