Let it set for a minute before you victim comes, and when he does, he might not notice it at first, but after a while he will.
This is my favorite toilet prank so far. When your victim flushes the toilet, the water goes down, but the water stays yellow. This prank will make it seem that the toilet is broken, or the pipes might be clogged. Another reason Why I like this prank is because you can do it in less than thirty seconds.