but I no longer recall if it works in both directions. If it does there is the potential for up to four bumper stickers of ever decreasing humor.

For example you could have:
1) a bumper sticker for the front of the car in red reading,
"If this bumper sticker appears blue, get the hell out of my way",
2) a bumper sticker for the front of the car in blue reading,
"If this bumper sticker appears red, call AAA",
3) an additional sticker for the rear of the car in blue reading,
"If this bumper sticker appears red, I've been abducted by aliens"

And that's on my first cup of coffee. :-)