You are still seeing the green curves that are on that column layer, yes. Curves do not have materials.
You could change the render mode so that it doesn’t render the curves in the scene.
I’m getting weird “Error: ‘NoneType’ object has no attribute ‘Document’” errors in this file, though. @pascal, any clues?
1 curve, 2 block instances, 1 extrusion added to selection. Command: Ungroup 1 curve added to selection. Error: 'NoneType' object has no attribute 'Document' Error: 'NoneType' object has no attribute 'Document' Error: 'NoneType' object has no attribute 'Document' Error: 'NoneType' object has no attribute 'Document'