Review for Account Colors by Sven Rieke
Rated 5 out of 5 stars
Since Border Color extension stopped working for higher Tunderbird versions, I was searching for a decent replacement, and this one is far better and complete.
I think it even has to much options, lets you tweak even more styles than I ever needed.
But now, since I upgraded to EarlyBird (TB 20.0a2), it stopped working too.
I hope there'll be a compatible version soon.
This problem is fixed in Account Colors 6.7.
To create your own collections, you must have a Mozilla Add-ons account.