I found this thread/forum after I received a replacement phone for my previous Nexus 5 that had nothing to do with the screen and just wanted to update everyone on Google's current stance toward the issue. They are NOT replacing phones that have the yellow screen.
After receiving my new (refurbished) phone and before sending my old one back, I had them both on at the same time and was backing up/installing all my pics, etc when I had the same wallpaper image on both and noticed a definite dullness and yellow greenish tint on the replacement phone. A quick search showed that it is a common problem but not affecting all phones. It is most apparent on white areas of the screen and very dull looking compared to my old phone. The serial number of the yellow screen starts with 04913. Sorry, did not think to check the older (good screen) before sending it back but it was purchased the first week the Nexus 5 went on sale from Google Play store.
When I called Google to see what could be done, I was told that this was considered a cosmetic issue and they would not replace it. The rep said he would check with the tech department and confirmed that this is only cosmetic, no replacement will be given. Seems they have either changed their stance since many others have been able to get replacements in the past.