A better way is to use the Reverse Faces and Orient Faces functions to ensure that only front sides of faces are visible.
Anssi,
the faces of the components are all oriented properly. This is a SketchUp bug that is rendering the section cut fills in the Back color randomly.
After doing that check the references list to ensure you haven’t added another .skp reference.
Happens sometimes. Something is not solid, or maybe move the cut a couple of millimeters one way or the other. But first double check that the wall is solid.