Many fish species go thru a "fading" or "peeling". When young the fish will be one color, then peel or fade as they age to their final colors.
When I got my Sagittae they were silver, with black bars. After peeling they were orange/white piebald.
I am not that skilled at determining if this is peeling or some kind of sickness. Maybe someone more skilled in that area will chime in.
What are your water peramaters?
Ammonia
Nitrite
Nitrate
Any other symptoms beside the color change
Tankmates, any new additions
Size of tank, water change/maintenance schedule