I'm trying to set up a layering system like in Photoshop using Mix Shaders for when I texture paint in Blender. But when I use a black colour on the 2nd/top layer, it doesn't show up. It acts as an eraser. Black will show on the 1st/bottom layer, though. Every other colour shows fine. I've also tried the same approach with a Mix RGB node, but still black does not show.