(Caution - flashing colors)
https://www.babylonjs-playground.com/#258JTK#13
I’m playing with linesMesh… and with the GlowLayer’s cool .customEmissiveColorSelector() customizable function (a continuous-running function). Five linesMesh, but all 5 are colored “selectively” with ONE glowLayer. Pretty fun… very neon-like… xmas-ish, somehow. 
It makes me think of a neon Santa… with animated waving arm (via turning on/off various neon at correct times). Can ya picture it? OLD STYLE animating.
Line 90 rotates the array of linesMesh every 1/10 second, which makes line 52 customEmissiveColorSelector() update the colors. FUN! My customEmissiveColorSelector() can be coded smaller, perhaps.
You say you can’t determine HOW the WHITE glow around the center linesMesh… is happening?
See line 77. Even showBoundingBox lines get “glowed”. Perhaps this is because… showBoundingBox lines are ALSO happening on the “emissive channel”, just like glowLayers.