I've got the same issue issue iPhine 6 and its driving me nuts. I'm coming from an iPhone 5s which had a slew of display issues, and was hoping to be spared with these on the 6. The top section of my screen gradually fades to a warmer, yellower colour. It also appears to warmer in the top right than the top left.
I had the same issues with my 5s. The first 5s I received had the exact same problem with a yellowing near the top; I swapped this and received one that wasn't so noticeable. 8 months later, the screen came loose and had to swap it. This new one had yellowing down the left side when held in portrait mode. That was swapped for one, but with yellowing down the right In portrait. At this point, I wasnt allowed to swap any more as I'd already swapped it twice for non problematic display issues. But, for being good customer, and trying to see me right, I was authorised by the store manager for a CRU - a Customer Replacement Unit - an iPhone fresh from the box, ssealed, with all accessories instead of the normal replacement stock. This too has a yellowing near the top, albeit sutbtly. Even worse, this unit has scan lines running across the screen in portrait mode. After 5 attempts, I went into the display lottery and came out a lose at the other end.
My girlfriends gold iPhone 6 also exhibits the traits, and when compared to mine, looks worse. The one I did check to instore seemed perfect, no gradient whatsoever. Additionally, another friend who has a Space Grey iPhone also didn't seem to have an issue.
Since I've noticed this, I've noticed ALL of my other devices have this yellowing, but much more subdued.
My work 27" iMac (2009) is yellow down the screen.
My 15" MacBook Pro with Retina display (2013) is more yellow down the screen, as well as to the left.
My iPad (3rd-gen) is also yellow towards the top of the screen, as well as the left hand side.
I'm guessing some form of discolouration is normal, and the idea of display panel uniformity is probably a pipe dream. The problem is, the brief times I did check other iPhones (apart from my girlfriends), I didn't study for long periods of time and instead compared them to the yellow area of mine. They probably exhibit the same issue, but the whole screen may have a different white point throwing everything off.
I'll probably try and live with it for now, I'm not going to rush to the Apple Store to swap it – I'll make an appointment for next time I'm passing by.
After the iPhone 5s, I must be flagged as a "problem customer" by now; but when you're spending £700 for a device you use everyday, you want it to be as good as it can be...