In all the Retina display devices I've had, I've never had or seen a perfectly uniform display in both backlight and tint. Each one always displayed a 'characteristic'. In my experience that was either yellow stains, blue to white/yellow colour shifts, dark patches or pink to green colour shifts (this is a slightly warped polariser in the screen apparently). There is no perfect screen out there. If you're unlucky enough to be able to see these characteristics, its about finding a screen that you can live with.
Take my 15" TB MBP for example. The screen is uniformly lit (yay!) and has no Pink>Green or Blue>Yellow colour shifts (yay!). The native White Point is much warmer than my 2014 13" rMBP but I have addressed that in System Preferences by making the screen temperature a little cooler.
What I have noticed is the tint of the screen changes at different vertical viewing angles. With the screen facing you head on, the top & bottom of the screen is slightly browner/duller than the middle of the screen. If I tilt the screen towards me the top of the screen is brighter and whiter and the bottom is browner and duller. If I tilt the screen back, the bottom is brighter and whiter but the top is browner and duller. This is in line with one of the comments made earlier in the thread by
DHagan4755 about the bottom 3rd of the screen being 'dusty'. That sounds suspiciously like what I'm seeing too.
You can easily see the tint change when scrolling this thread up and down and following a block of text with your eyes. I have no basis for comparison as I've never had a Retina display as large as the 15" before so I'm chalking it down to yet another 'characteristic'.
I've played the display lottery before and as the rest of my laptop is A1 perfect, I'm not going to put myself through that again. Its stressful and futile as there is no perfect display. You could end up with something much worse and be stuck with it. Convincing Genius' in store isn't always easy as these display issues are rarely visible in an Apple Store.
Here's my 2014 13" rMBP compared to my 15"
I think the photo shows the white point differences. On the 15" the top and bottom posts are slightly browner/duller that the post in the middle.