Water readings?
Any new inhabitants or other changes to the tank?
Is there a specific area on the fish going white? Is the fish breathing & swimming OK?
I hate to say it, but most of the time when a fish starts going white in an area, it isn't good news.
Old age can cause colors to fade out.
I've seen numerous fish suddenly start going white in patches and die within 48hours.
There are a few different diseases that have "white areas" as one of their symptoms. Bacterial & Fungal infections can cause white patches.
I had 30 rosy red minnows die in less than 36 hours due to what I believe was Neon Tetra Disease. White patches would develope at the base of the dorsal fin &/ or the tail. Then the fish would start swimmin spasmodically, then start showing breathing difficulties and finally death. The affliction killed the next 2 groups of mollies & platties I put into that tank too. I had to completely tear down that system, clean & sterilize everything as best I could, then let the tank N-cycle again before the deaths finally stopped.
I hope everything turns out OK...