Dual extruder together with luban

Hi, I have now the new dual extruder for A350 Snapmaker. I have a problem with printing:
I have a model (something like a stone) and I want only some parts of the wall in a different color. So I use the #Brush# Option to color some parts of the skin of my model. This works, but when I do the 3D print, the second color goes into the inner part of the model. This adds a lot of time to print the model because it creates bases and other structures inside the model with the second extruder.
How can I setup the settings, so that only the wall of the model will be printed with the second color (second extruder) and nothing inside the model?
Hope you have some infos for me.
Thank you.

Play with the different settings and types of painting geometry, there are better ways to do it also using a 3D CAD mouse helps

What kind of settings and painting geometry do you mean? Where can I find it in LUBAN?
Within the mesh Painting tool of Luban I can only choose between SMART FILL or BRUSH and some Tolerances.
But any change of this leeds to the same result: the second extruder prints bottom structures within the 3D object and not only on the skin of the Object. This costs a lot of extra time and material.