Iphone 6S Rose Gold has some worm running lines

Hi Briansyddall

This phone was in ultra thin black cover that is not loose, it is fit and tight. It doesn't touch with cosmetics.
Thank you for that info but I just want to know why this problem is causing to rose gold only. This iphone is my wife's and she does love iPhone more than me. She looks after it like a baby. Never dropped on the floor, never put out in the rain and never drop any liquid on it. So what might be the cause of this?
I use iphone 6s balck with the same cover but nothing has happen to it.

Thanks FelipV.

My wife buys Iphone rose gold only and all the rose gold she have used had the same problem. I'm using 6s silver now nd we both use the same kind of covers. But there's nothing wrong with my phone. Why only rose gold gets wear and tears?

Thanks for the info.

I would like to know why it appears.