As the originator of scale modding this guide is a 
re-release of my original scale coloring guide from early 2018.

For those who use Gen3/Bibo+/Vanilla faces:

1. Take the alpha provided and paste the texture into the alpha of your diffuse.
2. Color select all the white areas
3. On RGB channel paint the selected areas whatever you feel like. Color, design, etc.
4. Delete the alpha cutout 
5. Save and import your assets.
6. Done.

For those with custom scales:

1. In photo editing software open up a xaela skin diffuse or face diffuse.
2. Use color selection tool to highlight the skin.
3. Select the inverse of that to get the scales selected.
4. In the alpha channel paint the area selected white.
5. On RGB channel paint your scales.
6. Copy the alpha for Raen.
7. Delete the alpha cutout.
8. Save and import into your game.
9. Done