http://www.techradar.com/news/expect-some-color-shifts-and-burn-in-on-your-new-iphone-x-apple-says

Picked up a brand new iPhone X for yourself this morning? Apple has some advice about that OLED screen, namely that color and hue shifts, and perhaps a little bit of burn-in, are to be expected with this kind of display technology.

In a new support document, Apple says "you might notice slight shifts in color and hue", which is "normal behavior" for an OLED screen. Indeed, manufacturers have grappled with the problem in smartphones, televisions and monitors ever since the Organic Light-Emitting Diode tech first appeared on the scene.