How To: Preparing Double Layer Cutout Textures
The Problem: You have a single, solid texture and you want to assign some part of it a different color or material property (usually emissive) in your model JSON, while leaving the rest of the texture unmodified.
Solution: A simple way to accomplish this is by splitting the texture into two separate cutout textures, so that transparent pixels in each texture are solid in the other.
This article explains how to do this task using GIMP version 2.8. If you use a different bitmap editor this article will probably still be useful to you but the details of each step will be different. If you have never used GIMP and want to try it, you should probably start here.
This article uses the Minecraft vanilla Redstone Ore texture for the illustrations below.
Open your base texture in GIMP and decompose it into Red, Green, Blue and Alpha layers by choosing Colors/Components/Decompose... from the menu.
You should have four layers that look something like this:
