Rated 5 out of 5 stars

What I also found out is, that if you add a Lable to a specific e-mail (e. g label "private"), then normally after selecting that e-mail in the treadpane its whole line is highlighted with the color of its label (e. g. red).
However with account colors installed, the labled e-mail (line) in the thread pane is always blue when selected (as any other e-mail).

This review is for a previous version of the add-on (3.2). 

Problem resolved

This is resolved in Version 3.3 which works with Thunderbird 3.1.