Try this out guys put your phone in the freezer (yes that cold box in your kitchen) for a good 5-10 minutes while its ON.
Pull it out and leave it alone as it adapts back to normal temps. Open up a white background / app like gmail and you will see the yellow marks will be diminished.
I say this is a temporary solution because It might go back to the dark yellow caused by the adhesive paste under the glass screen.
This is the cheapest fix without having to replace the screen. glass.
I am no responsible if you freeze your phone