Russian Blue going bronze - anyone know why?

[MyCatIsTryingToKillMe]

I have two Russian Blues that are just recently turned two. They are brothers from the same litter but the bigger and more dominant one has turned a sort of bronze colour in places but particularly on his shoulders. I've goggled it but can't see anything where other people have had this happen. Anyone have any idea what might be going on? Photos attached so you can see them side by side for comparison.

I thought Harry was just odd when he shits on the manhole cover!

Sits! He definitely doesn't shit on the manhole cover!

I had a black cat as a child who also went 'rusty' if he had been laying in the sun often

My black cat goes more golden coloured in places during the summer. He's moulted and it's more apparent. Happens every year.

"I thought Harry was just odd when he shits on the manhole cover!"

He'll sit there all day now, looking sad, because of his accidental public shaming on the internet.