I have looked into it again, and basically at this stage I only have one App where I can reproduce this. In this regard, it could be also because of the app not working properly with the phone (software)?
With the app "Signal" it is possible to define which LED color should be used for the notification of new messages. Only when I choose the color red I get a LED notification, all other colors do not work.
I have tried the same app version on the FP1 of my partner, and also e.g. blue or green work there, on the FP2 however, only red.
As said, maybe it is an issue with the app rather than with the phone, nonetheless I have added it for now to the wiki on top, but please feel free to delete if you think this may be rather an issue with this particular app, or maybe someone has another app installed where you can choose the colors. I couldn't find a setting in Android itself to define the LED colors for notifications in general.
I also installed an app for testing LEDs, and also for that test only the color red works!
However, I have also tried to reproduce getting green light for a fully charged battery (following the description above), and it works: I can get a green LED when the battery is full.
So apparently apps may not be able to access other colors than red at the moment?