Why does my iPhone 4S's screen have a yellow tint?
It depends on the cause of the yellowing.

I personally have seen the following situations:


Psycological yellow where any time you have a black border and a white area, the brain will color the white yellow. If you hide the black with a piece of white paper, the yellowness magically disappears.
Permanent manufacturing problem where the LCD is pinched internally, damaged, or otherwise really emitting a permanent yellow cast as measured by a calibrated colorimeter such as the Spyder.
Temporary yellowness (very rare - I've just seen this a few times). I don't know why or when it went away, but several people sure agreed that one phone was off until it was restored. I don't know if it was some odd warmup temporary hardware issue, but after it was restored, it looked and tested fine with the Spyder.


Do keep in mind, not all displays are made in the same batch, same factory and that combined with the backlight settings can make a pretty large difference between models. If it's convenient to have it checked out, or you can take a picture of the colors and it's totally obvious yours has some severe color tint, then take it in and ask for a repair. If not, you'll likely need hours of time and expensive tools to even tell your phone is unique to another after using it to play games for a day or two. It will just be right and your brain will compensate for whatever calibration your phone has. A restore of the device is also a good thing to try so you don't needlessly waste time going in for service since that is the most obvious cause of the LCD passing all inspections along manufacture but you having colors that seem off to your eye at the point of receiving a new phone out of the box.