Thanks for the fast reply.

unfortunately it was a little bit to advanced for me.

So, I have the population and within I have an Object Maker and a Density Shader. In my case the input for the latter is a surface layer which has a Blending Shader input. And you suggest, I create a Painted Shader and connect it to that input?
And another question: For testing I have just created a Painted Shader but when I open it and switch to Paint Mode I only see a gray canvas. Although I set the view to the current render camera it remains gray. How am I supposed to see where I am painting?

And I wasn't able to find a kind of "invert" button for/within this shader.
Or am I just doing everything terribly wrong here?