The plan is to get two pieces of sturdy paper which are the same color as the victim's ceiling. Tape them next to each other, with the outside edges taped to the ceiling over the victim's shower. Use a weak piece of tape to connect the two pieces of paper.
Then run a string from the far side of the tape to the towel.
Put a bunch of flour on the paper.
It may take some figuring to run the string in such a way as when the towel is pulled on the center tape is removed, dropping the flour on the victim.
The result is: the victim
- takes a shower
- is wet
- grabs a towel to dry off
- gets covered in flour, which sticks to the wet person
disclaimer: I have not tried this. Also don't do it. After you've finished not doing it, tell me how it went.