it's the glue used to affix the LCD panel to the body of the phone.
when it heats up, it tends to leak onto the screen.*
LG reportedly replaced panels for those devices affected, with a new digitizer and panel, but it all depends on the workmanship of the assembler.
Sony apparently now uses glue that can withstand upto 70 degrees*
Sent from my HTC One X using Tapatalk
Another maybe potential fix for the yellow tint:
This is the only bad advice I have ever seen on WHT, in fact I signed up just to post this.
I have 3 Nexus 5's in my household, I noticed the tint on the second one. Google offered me a replacement but I read that it could be the glue used on the screen. I still don't know what causes it but all you need to do is turn your brightness up to full for a few days.
On the second phone I turned the brightness up to full and left a video running overnight to make the screen hot. The third phone I just turned the brightness up and let it clear in it's own time.
All 3 phones now have lost their tint. The second one obviously cleared quicker than the others.
Sent from my HTC One X using XDA Free mobile app