When I have the glow effect, it is the color of the texture of the mob (or player). I have tried to use the glitch with optifine fast render to make the glow texture solid, and it just shows the back of the texture of the mob or player.
Without fast render, but you can see the texture


With fast render, much more apparent

As you stated, it's an issue with Optifine. In your description, you didn't state anywhere how Metamorph is related to this issue. I don't really think it's on my side.

Before I installed your mod I had the glowing effect and it would function normally. When I have your mod, any colors that my glow effect would normally be was replaced with the colors of the model I was, this applied for all morphs. This was especially weird because I was able to see out of the back of the ender dragons head. The reason is that I mentioned optifine was that the fast render is incompatible with the glowing effect, and it better displayed the problems