I had the same issue. Bought my brand new iPhone 7 plus, which had a scratch on the screen so sent it back. I didn't get to see what the screen was like as I didn't turn it on.
New replacement arrived, no scratch so put tempered glass screen protector on to avoid scratches then switched on and began to set it up.
At this point I noticed the yellow tinge to the whites compared to my iPhone 6 and searched google where I found all these hundred of similar complaints.
After a long search and visit to phone shop to compare with display models I discovered my issue.
I removed the glass screen protector I had fitted from the start ( ESR tempered glass with 4.5 out of 5.0 on amazon - 265 reviews) and Placed it on just half the screen, so had half with protector and half without.
The screen protector half had yellow tint and half without was perfectly white.
After searching more into the screen protector itself on other sites I found reviews saying it has a yellow tint to it.
Problem solved for me.
Thought I'd put this up in case like so many other people some of you may have put a screen protector on your new phones from the start.
Let me know if anyone else has discovers this.
Cheers.