Illness/Sickness: Dull skin on Marbled newt

I have noticed a change in the skin color and texture of one of my Marbled newts. It is considerably duller than normal, and the texture seems to have changed. In addition, this newt appears more lethargic than usual, and sometimes lies out in the open for hours at a time. At first I thought the lack of activity may be tied to the cold weather, but the other newts don't seem to be affected.
They are kept in a densely planted terrarium. I have had them about five months. Current temps are low to mid fifties. With the cold weather, they are less active than usual, but otherwise appear to be feeding and healthy.
I read that they can get nitrate poisoning from substrate if it is not changed, and that can present as dull skin. Could that be what this newt has, after only five months? Or are there other possible causes?
The afflicted newt is now quarantined, I'm waiting to see if it clears up.
Thoughts are appreciated.

That young of a newt probably would not put off enough waste to poison the substrate. You can use bioactive substrate (springtails and dwarf isopods) to help keep the substrate clean.

What foods are you offering the salamander.

They're eating crickets, chopped earthworms (euro. crawlers) and tiny dubia.
I do have springtails in there also.
They have grown considerably in the time that I've had them.

Maybe he is getting ready to shed?

Maybe- but it's been like this a couple of weeks. I'll keep it isolated for a while, maybe the condition will clear up.

I have a similar problem.
I bought larvae of Marbled newts from two breeders.
One group of them became as dull as yours and another became vivid green.
I reared them in almost same condition.
I wonder why.