Illness/Sickness: Axolotl losing colour?

I own 2 adult wild type axolotls. I'm a first time owner, they were originally in a zoo but were going to be put down (there were originally 3, one of them died of stress from people tapping on the glass, and they couldn't keep them if they weren't on display) So I begged my mum to let me adopt them, but I've had Batman and Aristotle for about 5 months now. There's been a few minor hiccups, some scraps and a leg amputation on batman, other than this they seem to be doing fine. But I've noticed that Aristotle seems to be losing his colour, he was paler around his belly and tail than Batman when I first got them but now his belly is almost white. His legs, back, tail and face havn't changed much, he's gained some more yellow "freckles" but that's it. His behaviour hasn't changed, he's eating and normally and swims about the tank happily. I've tried to research the topic but I can't find anything similar, I'm not sure whether its because they are quite old, they were roughly 12 when I got them. Batman isnt affected and Aristotle seems to be ok otherwise, I just wanted to know if anybody knew anything about this

photos of Aristotle before and after

I can't open your pictures, but it sounds very much like vitiligo - a condition where they lose their pigment. Harmless, but if you've not seen it before I can understand your concern.
I have one of these too - started out as a lovely wildtype, now looks like a leucisitc.
Think of it as the axolotl equivalent of going grey.