Apple India — iphone 7 plus poor material quality

I have an iphone 7 plus 128 gb in matt black color. The phone was replaced due to manufacturing fault about 2-3 months ago. After that from 1-2 weeks i am noticing discolouration of my back panel. There are no dents or impact damge as i keep my phone with atmost care. I visited the service center and they clearly refused coverage in warranty. I called apple support and they said whatever store says is true. Am i a lier in view of apple? I need a solution immediately or else i will lodge a criminal and consumer case against apple.