Subject line in red? Really? That's rather unusual. May we see a screenshot? Addresses, yes. The design intention is that if an address you write to is shown in red, then Thunderbird hasn't been able to find a match for it in your Address Book, and is trying to alert you to this. You may have mistyped a known address, or you may appreciate a prompt to store this new address in the Address Book. Having said that, Thunderbird seems to have a bug whereby it paints addresses in red even if they are known. Be aware that your correspondents may not see decorations in your message such as changes in colours and fonts. There are well-established conventions for distinguishing replies in email which don't rely on optional features such as colour. Bear in mind also that hard copies of your messages may not always be printed in colour.