Sounds like a faulty display to me. If the results were the same on an external monitor then the issue most likely pertains to the GPU, but because an external monitor displays a perfect picture it has to be an internal display issue.
You can get a replacement screen [https://www.laptopscreen.com/705358/|here]. The only option available in LCD format is a glossy finish.
The procedure for replacing the screen should be similar to the one in [https://www.youtube.com/watch?v=qQKo2v5Hes8|this video.]