While the Thunderbird folks are finally making their "account colour" implementation more complete in this ticket, they are discussing amongst themselves whether a colour indicator should only be added to the message list (also called thread pane) for the beloved (irony!) cards view, the one that at default values wastes a lot of space, or whether it should be added to table view as well.
So one developer says to the other (quote):
Let's do this without [table view] and see how many complaints there are.
Surely user complaints are a good design principle.
Needless to say that Betterbird has shipped full account colours for both views since Betterbird 115.6.0 and even full row colour since Betterbird 115.8.0, so for more than two years. And those folks are still discussing whether a partial solution will do.
UPDATE 12th June 2026: So the feature got included in Thunderbird 153, currently in Daily/Alpha stage, going to Beta next week. As mentioned, an account indicator is only displayed in cards view and in the folder list. Apparently, the Thunderbird folks are seeking more complaints, since all people who haven't configured account colours will now get a rather useless additional blue indicator showing the default colour:
(Screenshot taken using some newsgroup content.)
And in the folder list:
How useful is that?
In Betterbird, we're maintaining the indicator, but we're not showing it, when no colour is set:
