Why is my screen displaying weird colors

As you only see the problem within the internal display and not the external that proves the issue is within the internal displays logic. This is in fact a common issue when the T-CON board starts to fail from overheating. Apple did a bone heat design here pulling out the T-CON board from within the display lid placing it nearest the hottest area inside the uppercase!

This often happens when you are running your system hard and/or the vents are blocked and/or the fans/heatsink is loaded up with dust so the system is unable to cool it’s self.

Time to open the system giving it a good clean of surface dust on the logic board and bottom cover using a small soft paint brush, then taking out the fans to likewise cleaning then, and lastly cleaning the fin areas of the heatsink using short blasts of can’ed air and giving the other area the same treatment to push the junk out.

Then put your system back together to see if that was all it needed, you might want to install a good thermal monitoring app like TG-Pro which can also push the fans up when the system is running hard (Apple doesn’t push the fans hard enough).

Sadly, if none of this helps you need a new display assembly MacBook Pro 15" Retina (Touch Bar, 2018-2019) Display Assembly and here’s the guide Remplacement de l'écran du MacBook Pro 15" Touch Bar 2018

FYI - The metal covered chips on the circuit board the cables from the lid connect to is the T-CON board which fail.

The UI parts look absolutely fine so I don't think it's the screen itself, it's most likely a bad GPU or more commonly, a software issue.