Betta Fish: Betta colour change

First 2 photos – tonight

Next 2 photos – 3 months ago

Last photo – 7 months ago when I got him.

I know they have marbling genes that can change their colouring, he turned all black pretty quickly after I got him. But over the weekend his gil on one side seems to have gone grey. It’s not fuzzy, just like he’s losing his colour. It doesn’t seem to be a normal colour change. A couple of his back scales have gone grey too. He’s behaving totally normally and eating etc.

Should I be concerned?

Tank parameters:

0 ammonia, nitrite

10ppm nitrate – just done weekly 25% water change.

Ph 7

Temp: 27c

He lives with some cherry shrimp and tank is heavily planted with lots of hiding and resting places.