Do you sometimes put your phone in places where the temperature is too high? Or under the sun for quite a while (like an hour or so)? Excessive heat could burn the screen, causing part of screen turn yellow permanently.
If it bothers you, return it. Keep in mind there is no perfect LCD screen, just some are better than others.
I think I shall return. What do you think about the picture i posted? Its bad, huh?
I can see that it is not perfect. The top left matches what I see on the bottom. The screen is warmer in places and yet cooler on the right side towards the top.
I am not sure I would trade for this but like I said, if it bothers you, return it and try again.