Here's the Good news, their is nothing wrong with your iPhone, it's absolutely fine. That would have resulted due the use of adhesive on the display, and some iPhone in this category, talking of the iPhone 7, it still needs some little settings, and in no time that should be out of your screen.
The tint can also be from app softwares on your iPhone, like i said earlier, it needs only some little settings.
Here's what to do,
How to remove the Yellowish Tint you see on your iPhone 7 or 7 Plus screen
Head over to your phone's general settings, next, click on Accessibility , display Accomodations, Color filters and toggle the switch to ON (Before it was OFF, turn it ON).
Like the screenshot above from Osxdaily, it shows that you can be able to change the screen's color to your preffered choice.
Now, to get rid of the yellow tint, slide the Hue bar around till you get the color you want, and that would be all. You can now say "Good bye" to the Yellowish Tint.