Hello! If that area stays yellow no matter what windows are open and never changes its a hardware problem. I would contact tech support for warranty assistance.
What is the exact system model? When did the issue start? What is the current OS installed on the system?
Check if the issue is seen while you perform the below steps:
Restart the system and press f12 key on startup - choose diagnostics. Let it run the diags and if you encounter any errors - make a note of the complete error and report the same to us.
Run the LCD bist - Turn off the system and press and hold the "D" key + power button to turn on the system and it should run the LCD built in self test - see this link for more info - http://dell.to/1SkXNrG
Restart the system and press f2 key to enter BIOS - check if the issue is seen while in BIOS as well.
Connect an external monitor and check if the display is fine on the external monitor.
Based on this, we could isolate the source of the issue.
Also, please click my DELL-username and write me a private conversation with the service tag and your contact details(Name and Email) for case records.
Let us know if you have any other queries.