Hi,
First you can try to open the phone and carefully clean the LCD connector and the Flex cable. But based on my experience, I think the problem is in the display.
Display operates on the principle of a matrix. At the edges of the display there are many lines which are commanding points on the display, line by line. If there is a slight rupture, then some of these lines do not work permanently (depending on temperature, pressure...). If you have abnormal color, this means - missing color.
Another reason can be, a shortage in power supply to the display.
A third reason, may be depressurize between the layers of the display. But then you will have a growing local spot.
This phone use IPS technology - more flexible then AMOLED which makes it more resistant to shock and pressure.
Try pressing the display (not very strong) at various locations along the edges and see if some lines will disappear.
But you can found the problem only by testing the phone with another LCD display.
Regards.
P.S. My English is not very good and the problem is difficult for explanation. You can found materials for the IPS technology in the internet.