Sure, I'll add mine. I once had my characters enter a dungeon with red walls. I kept stressing the red, in every scene. "The walls are still red. The corridor stretches for about 15 feet. The walls are red. Like blood. Red. Dead. There's a door at the end of the corridor. It isn't red. Red. Red." Occasionally I'd interrupt one of the players to say, "The whole place is still red." It drove them absolutely crazy.