I didn’t find the black ones worse than the white ones. In fact the ones I remember best were white as it happens. I agree about the sound of the slipper. That is something that made a deep impression on me. The slipper produced a loud room-filling thwack each time it landed on someone’s backside. The sound reverberated around the classroom. Maybe it sounded louder because the classroom was otherwise totally silent during a slippering. Some of the walls were quite thin and we could sometimes hear a slippering in the adjacent classroom.