I was having the yellow screen issue as well.... I went to my local Apple store and got the run around, they told me they never heard of any such "problem" and said everthing was fine
I then went on vacation to Chicago and finally talked to a rep there.... He looked at my screen and said everything looked fine.... I then finally got him to compare mine to the floor model.... He was shocked by the difference and suggested that I make a genius appointment as soon as I could.... I return the next day and spoke with a genius who said she never heard of this poblem, but then compared my phone with another and instantly said.... Something is wrong.... She replaced it there with no other questions...
So as I speak, I am on my "new" 3GS..... The color tone is much better....
Try your local Apple store and insist on getting yours compared to the floor models