$brightness

I dont know what this is but i saw it in the VMT for sparks

"UnlitGeneric"
{
	// Original shader: AddTexture
	"$basetexture" "effects/spark"
	"$brightness" "effects/spark_brightness"
	"$additive" 1
	"$vertexcolor" 1
}

• 

  spark.vtf

• 

  spark_brightness.vtf