Having two different wall materials, with graffiti on both I am working on an abandoned hospital. I would like to add graffiti here and there on the walls. The graffiti is a PNG with alpha. 

I have managed a rudimentary look and feel on a test piece, to be refined a lot soon, but the graffiti seems to make the materials under it transparent too, resulting in this type of image:



So the question is, what should the node system be to allow three materials on one piece of wall?
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The most common way to set this up is to have 2 shaders, one for the wall, and one for the paint.

Then, add a mix shader with both the wall and paint shaders connected. Then, use the alpha output of your image as the factor.

It would look like this: