What was the color of the wallpaper in the Twin Towers?
... plane.
Twin Towers Wallpaper
Explanation
Explain Bear
Listen up, buttercup! This "joke" is dark because it's about the Twin Towers, which got clobbered by planes on 9/11. The punchline is "plane" instead of a color, get it? It's funny 'cause it's so wrong, like you trying to parallel park.