Material overlay

Another time working on this material and another problem appeared.
As you can see on the screenshot of the node editor, i have 3 materials
1 - blue car paint
2 - clear metal
3 - red color material

And my question is: What factor should i use in the last mix shader node to make that black color be material number 3?

If i use “Col” as attribute, can i specify which color i want? Like i want red color in vertex paint mode to be my red material and blue color in vortex paint to be blue car paint?

Well, kind of. You could do that and separate by color and use that as a mix factor. More common is to use two different vertex color layers, each of which is black and white and then chain them using two mix shaders.

Tip! when you create a second vertex layer in the data tab (the triangle) it creates a copy of the previous layer (which I don’t like) To set all the verts of the current vertex color layer to white (or any other color) change to that color and hit shift k.