Paint on the Colormap?

Hi everyone, I’m a total beginner who’s building up his first map. I modeled the terrain in World Machine, and exported the Highmap and the Colormap.
Then I imported in UE4 the Highmap and created the landscape, and finally set up the material using the Colormap.
Here it is (not finished yet)

But there are some imperfections that I’d like to fix using the paint tool.
I found this:
but I’m not sure I can do what I want in this way (The layer info would be something that “overlaps” to the splatmap), as a matter of fact when I try to paint on my landscape, the texture is applied to the entire component.
So… What have I to do in order to paint on my landscape, on the existing Colormap?