Line color priority is one of per-material line color settings introduced in a recent commit (7915d7277ac8). This parameter allows artists to fine tune line colors at material boundaries. The present blog post is a quick demonstration of the new feature.
The first image below suffers from a visual artifact that alternate line colors appear at material boundaries. This problem is caused by the lack of information based on which one of the two materials could be chosen to determine the line color at material boundaries.
Line color priority is intended to address the documented issue. Specifically, line color priority is an integer value (zero by default) specified for each material, and is used to pick up the material having a higher value at material boundaries.
The second image below shows an improved result where the priority values of the blue and white materials were set to 1 and 0, respectively (see the third image below for line color settings of the blue material). Since a higher priority was given to the blue material, now lines along material boundaries are always drawn in blue.