Switchable layered overlay textures (ie tattoos)

I have this character model for which I’ve created a couple of tattoos/overlays that I want to have integrated into the shader. Nothing multi-colored; just black over transparent PNG files. I’d like to find a neat way of being able to switch between them while also adjusting the color, transparency, and emission of the overlays.

To give an example of what I’ve been doing to achieve this so far, it’s basically like the process described in the first three minutes of this video: https://www.youtube.com/watch?v=nmzAbeUDPUQ&
…Except I am mixing the textures through the alpha outputs, not color (it’s given me some patchy/weird results). So far, I’ve managed to get it to display with light color values, but it disappears when I try black or any dark colors. It must be getting mixed up with the transparency somehow.

Any solutions, or perhaps better ways of going about this?