We lately discussed a similar issue at stackoverflow:
Short answer: You can render your line segments with an additional color geometry attribute and by setting Material.vertexColors to THREE.VertexColors.
@miguelseguramx
Hi!
You didn’t add colors to the new edges geometry.
If you change color of scene’s background to something brighter, then you’ll see edges of black color.
Or you can remove vertexColors: true from material, and you’ll get the edges of white.