iPhone 6 Plus color uniformity...

Wouldn't surprise me. Every iPad Air I have used had screen uniformity issues.

 

My screen is uniform. It's a cool tone - more blue tint if anything, not yellow. I just stared at this forum on there an a bunch of other things trying to find it but it's not there.

 

I have color uniformity issues on my 6 (non-plus). It's yellow/brownish in the bottom-left and a bit in the top-right. It's especially distracting in the Mail app since either the left side of the keyboard looks brown or the bottom couple of messages in the list look "dirty". The title bar looks grey on the left and yellowish on the right.

As you say, not as apparent on a fully-white screen. I took a couple comparison photos against my 5S and the 5S actually looks worse on a white screen but for some reason looks better when I'm using it (never noticed the problem on the 5S until this test; immediately noticed this problem on the 6).

Apple seems to have a really hard time producing displays with good color uniformity for some reason. Even my MBP Retina has this issue (but to a less distracting extent; also confirmed with a colorimeter) and this is my second retina MBP -- the first one I returned because the LHS of the screen had a pink hue.

 

The bigger the screen, the bigger the chance of variation across the panel, and the easier it is to notice any variation.

 

I have the same issue, but I afraid that a replacement will be the same one