@Ingo Thank you for the ideas. This happens at random when the phone is switched on with screen off and I switch on the screen. Then, the colored noise can persist even after reboot.
Because of this pattern, I didn't identify any app as the troublemaker.
Thank you for the fairphone community map.
Since following 'https://support.fairphone.com/hc/en-us/articles/115001041206-Troubleshoot-your-Fairphone-2-issue > Display > My display shows a pixel image (coloured noise)' leads to the exact picture of my screen, I suppose this is most probably a known hardware issue.
I followed the instructions on @advocatux link, which means opening and cleaning every inner parts of the phone.
It works again but it has already been doing working - colored noise - working loops so I will keep you informed.