Not sure if thats genetic but surely these fish do change colors with mood...They got a dark phase also where half is dark brown....I’d say this is normal/OK
I had a Foxface for quite a while and he grew quite large. Yet he was a big sissy, and all the other fish stressed him out. Consequently he hardly swam in the open, and most often took on a typical ‘bruised banana’ look— their idea of camouflage.