I am still using El Capitan, for several reasons.
So, in Mac Mail, I have the option of six banners of different colors. I want to be able to mark emails more than once, that is, with several colors.
Actual use case: I run an online literary magazine and I am marking emails such as Orange (to read), Green (accepted) and Red (rejected). BUT I would love to be able to add a gray label to accepted and rejected emails (for example, green and gray – accepted and published, red and gray – rejected and notified to the recipient). Anyway, there are multiple cases in which this would be useful.
Oddly enough, when I sent emails from one place to another, I have seen several banners! But they are not in the usual place (left side of the user's email, top email) but to the right of the user's email (bottom email, green and gray!) I have no idea how this happened and I haven't could reply but it's exactly what I want