This looks like one of the color blindness filters got activated. Here on Plasma 6, it's a Desktop Effect in the System Settings. Not quite sure, if that was also already the case in Plasma 5, but if you just type "blind" into the search bar of the System Settings, then it should show up and you can disable it.
No option on plasma 5 that I could find, I learned that it was a graphics driver that updated and caused all this, thankfully I had automatic backups setup.
![[Solved]What happened to my background/desktop?](https://lemmy.world/pictrs/image/d18231de-d9d4-479e-9a62-165c02174d9a.png){kind=link}