I am having the same problem. Bought one iPhone 6s 64gb space gray. I don't have an Apple Store near me, so I am currently on my second replacement. They will probably send my third phone tomorrow. I won't accept a $800 phone with a screen like that. This is awful in every light tones screens like white. And all the iOS is white on the top. If my third phone comes with this horrible screen, I am going to ask for a return, refund, and sadly buy an Android phone. I currently have an iPhone 5 and didn't wish to change from iPhone, but Apple is obligating me to do so with this lack of respect with consumers. I can't believe there people that simply accept a screen with such an annoying disuniformity.
I live in a place where there are no Apple Store near me and I got my phone via a carrier and a contract. The problem with the display is I went to the authorized resellers, checked their units and they have the same unevenness on the top of the screen... :/ I am impressed with other aspects of the phone. It's unlikely I'll get one with a better display and I don't want to get a refurbished model. I guess I'll try to change my phone during the end of it's warranty.. and live with it for now.