Hi, xxx, this yellow tint is never a defect.
Find 7a are using LED Backlight Design, hence brightness is higher at the bottom of the screen, in order to evenly distribute the brightness,
we set the transition area at the bottom of the screen with gradual darkening effect.
Find 7a uses the finest and most renowned Screen Supplier JDI ( JDI is a Screen Manufacturer)
Considering to obtain the Best quality Colour reproduction , OPPO choose the Warm colour Tone.
Therefore, on a white colour screen, there will be a slight tint of yellow on the screen.
If fact, if your iPhone 5s is not that small. The same yellow tint will appear.
So far, JDI hasn't found the balance between the tint and the compromising of the screen performance.
Hope this answers your query