My understanding is that is undertaken with an add-on. Have you approached the add-on author?

Hello Matt

Also with the add-on "Calendar Tweaks" I get no color printing.

Color printing worked fine with thunderbird version 52.x.

Long ago, I had this color printing problem already with thunderbird. At that time I got a recommendation to change something in the printing part in the thunderbird configuration file. But I don't know what I had to change at that time.

Regards Hermann