This covers it https://support.mozilla.org/en-US/questions/1161010

I have been a user of Thunderbird for many years and am extremely happy with it, except in one very minor detail. I migrated to Thunderbird after many years with MS Outlook Express. In OE, it was possible to automatically have the colour of reply text in a preset colour, usually blue, but this could be changed. Does this option appear in Thunderbird settings? I'm aware that the reply text colour can be changed by highlight and the colour change icon, but this seems an unnecessarily long way round to do something that could be set to happen automatically. Am I missing a setting somewhere?