alfani:
In your top image (the only one where the slab is not selected), there appear to be black dashed lines over the slab outlines which look like they are the correct color. Are you sure there are not other elements overlaying the slab in section? You might want to look at the slab in isolation from all other elements.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David