I'm on my Third iPhone 6 now. 2 Space Grey, 1 Silver.
I had a whole other problem which prompted the colour change, involving the grey wearing off the camera bezel and bottom of the rear shell, along with scratches all over the screen.
But concerning the backlight uniformity problem I know for a fact my first Space Grey had this issue in the top right and the Silver I have now has it a little worse in the same place (The second grey I returned immediately due to a loose touchID button so didn't have enough time to evaluate the screen).
The first Grey I had for around 4 weeks and I noticed that the darkened corner very slowly faded to become closer in uniformity to the rest of the screen, this is also happening with my 1 week old Silver model. I do believe it is a problem with the glue in the screen, perhaps where the machine finishes the application and leaves a slight excess which obscures the screen a little whilst still tacky.
I've heated this Silver model with hand dryers and hair dryers, this seems to have sped up the drying process (Do be careful though as the hairdryer I used got so hot that I could see the glue melting over the LCD and moving around, this returned to it's normal state after only a second of removing the heat).
All in all, I think this problem will slowly resolve itself, I remember my first iPhone 5 (Silver) had this issue which I either completely forgot about after the first month or more likely it dried up and disappeared so I never had to worry about it. I've noticed looking back over the forums that the iPhone 4/4S/5 and 5S all have threads relating to this issue immediately after release. I say give it time, if it doesn't go away in 2/3 months then go to Apple, being hasty about it will only end up with the same problem whilst production is still rushed and perhaps additional problems.