What's wrong with this display?

My display was working all fine until two days ago. Suddenly, the colors started looking bleached, the contrast is weird and the display will show traces of things that aren't on screen anymore.

Other things I tried: when I plug an external display the second display works fine, but the primary screen stays bad. I tried to reset SMC and PRAM already.

Pictures:

Login screen: you can notice that the image looks weird. The "enter password" field is less readable than normal.

If you open TextEdit, which has a white background, you can see a "ghost" of the login screen:

Other windows, and specially shadows, look weird, the image has some stains:

Question: is it the case of changing the whole display (expensive!) or could it be just some connection cable?