Unscrew the showerhead and insert several tablets of blue dye. Easter Egg dye might work, but you want to make sure that it's very concentrated. Also, if possible, find a dye capsule that takes a couple minutes to dissolve before releasing the dye. That way, in case the victim turns on the water until it heats up, they won't notice the color. Then, when the dye is released, it will cover them head to toe in blue dye that shouldn't wash off for a day or two. If you're lucky, they'll have their face in the water with their eyes shut, and they'll be extra smurfarific!