I am also saying one big YES for this - the add-on account colors is really important if you manage more email accounts, only my personal emails are reaching number of 12, then I have to also take care about non-profit email, so its really something important. Maybe much more easier can be if we can able to set background for each email client, like I can do in gmail on web.
But sure I am also ready to kiss whatever as* to make this come true :))))
Colors work very nicely on K-9mail which I understand is linked to Thunderbird, feels like this is a necessary addition to any email client that supports unified inboxes.
I know that kudo should be enough, but as a regularly donating long-term user I also want to cry out with a comment for the revival of the Account-Colors features! Thanks for the developing efforts and for considering to re-add this.