I have this same issue. This is my 3rd replacement. All had the screen issue and others had some build issues. I spoke with the apple specialist assigned to my case today and he wants to wait about a week or week and a half before replacing it again. He is adamant that it is an adhesive curing issue. I am absolutely convinced this is a backlight issue as OP pointed out.
I know one thing for a fact. I went to two different carrier stores. Their white iphones all had this issue and the space gray ones did not. I explained what I saw to the apple specialist and he states that this is only noticeable on the white cased iphones because of how stark white the face is. I don't think this is the case. All the space gray phones I looked at have a very "blue" screen and not warm and yellow like my current 6.
In a week and half when this issue still exist -- I will be calling them back and asking for a space gray replacement. They also have no interest in this thread or photos for evidence of the issue.