This extension is clearly aimed at containers and containers aren't meant to work on Private Windows - https://support.mozilla.org/en-US/kb/how-use-firefox-containers - so I don't share your view on considering an issue that this extension doesn't work there.
Also you say that the colour in this Private Windows should be Purple but that's one of the possible colours to use on containers (in non-Private Windows), so it could also cause some visual constraints if one person was to have a open Purple Private Window and a open Purple Container.
If it works good overall, I don't think it deserves 4/5 stars just based on that but I'm glad you found a workaround for your specific use case.
