Mine is ever so slightly warmer at the bottom. It's hard to see in most situations. I suspect very few screens are perfectly uniform. Apple just doesn't make screens for perfectionists. Most people don't sit down for hours and inspect their screens. haha! Maybe one day the technology will be so good all the screens will be perfectly uniform, but we're not there yet, at least not for a reasonable price.
If your screen is so not uniform that it's easily noticeable, then return it. If you have to sit with a white background and squint, then that's probably as good as it gets.
Yeah, after looking at my iPhone 5s and 5, I noticed that they also had a slight warmer tone ...but at the top of the display! I actually did not noticed it until now, 3-4 years later