Dynamic render with paint splatter effect

Hello recently I came up to this website: [http://retroepic.com/paint-splatter-surface-effect/][1]

What I want to do is create that effect which basically shows another texture instead of the default texture in a plane but only in the parts that the "paint" splatters.

I came up with a pair of ideas that were posible but I would love to hear your ideas and how to do it. Of course I think I'll have to download the Pro trial for this.

My first idea was to use the RenderTexture class which I heard in another thread and that would help to dynamically change the texture but I'm still not sure how to complement it with the paint effect. Secondly, I thought that maybe shaders can be used to with "culling masks" to oclude the texture in certain parts of the texture.

I guess my main problema is with creating the paint effect and detecting in which parts it has painted the wall.