It is not true that all iPhones have yellow screens. I have compared my old iPhone 4 to my brother's and friends' and theirs was much cooler in color compared to mine. I returned it and my current iphone4 screen is awesome. The colors doesn't look washed out.
Did you compare with another iPhone4 ?
Do you have nay pictures to show us the comparison? You would certainly be the first.
All YouTube videos, pictures of iPhone4 alongside iPhone 3GS show the i4 slightly warmer.