Rated 3 out of 5 stars
In Thunderdird 66.0b3 (64-bit) with this addon enabled mails with assigned labels are not colored.
When you deactivate the addons you see the colored emails with their labels, it is a "visual" error.
Fixed in version 3.1.5.
Thanks for letting me know, but please use github (https://github.com/jdgeenen/gcontactsync/issues) or the forum (http://pirules.org/forum/index.php) for reporting issues in the future since they allow a better dialog between us.