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Sorry, I have tried all you suggest but all events still stay green!

Not sure what you mean by 'Or use Australis ReDesign TB without Lightning-Design, when you use Calendar Tweaks !'
How is this done?

Questa recensione riguarda una versione precedente (31.7.5) del componente aggiuntivo.  L’utente ha inserito altre recensioni su questo componente aggiuntivo.

In Thunderbird:
Under Settings - Calendar - Generally.
Click on:
Optimize colors for ... ( it must set / check mark ).
Then you use windows color.

If this not work, then use Australis ReDesign TB without Lightning-Design ( Version 30.31, Version 27.5 ).

I use my own theme, it works, but i don't use Calendar Tweaks.

Colors for category and colors for events, i can change and setting !

- see screenshot:
green = Feiertage ( Holidays )
blue = Privat ( Private )
yellow = Termin ( Events )

Last try with version 31.8.5 ( download from the cloud )
or 31.8 ( without Lightning-Design )