This war paint works by replacing several unusual textures.

It replaces both "wearblend" and "albedo" textures for each weapon it's appliable on. The "wearblend" texture is a texture that essentially judges how a texture is worn and how strongly. The "albedo" texture is the texture that shows what the weapon wants to wear into.
By putting the lava texture on both textures and aligning them together, you can achieve an effect like this, where the war paint will not wear with scratches, but instead into lava cracks.

The lava shines more than the stone because it's brighter than it's fellow texture in the alpha layer for the albedo texture. Because it's more brighter in there, it essentially has a stronger reaction to light.