Tenhi seems to have been right. Moisture.
My brother assumed it was that so he put his phone in a bag of rice for a few hours and it got slightly better. But since he bought the phone from ebay and it has no warranty, he followed this guide to open the screen and wipe all the plastic (or whatever) layers behind the screen (the reflector, diffuser, and lenses or whatever, see the video). Then he left only the screen part in the rice bag for hours and now the phone is as good as new (almost).
It still has kind of a violet tint in the middle of the screen, if you look at the phone from the side but from the front it isn't evident at all. The screen also had green edges that disappeared.
For anyone wanting to attempt this procedure I would suggest only doing it if you have no warranty and if you have some experience in opening up a phone and/or feel confident about it. Also simply putting the screen part of the phone in rice would probably suffice, there probably is no need to take out and wipe all the 'plastic' layers behind the screen, I am thinking.
Anyway, you can see the photos of the phone attached.