Unscrew the shower handle and put red food coloring inside just before she showers. Preferably use the type that's solid and needs a few minutes to dissolve in water.
You could also mention beforehand that you think the water smells funny.
(Just kidding, don't try this unless you want to be evil)
A prank should be funny and enjoyed by the person getting pranked.
OP.
Using things she's deeply afraid of to "prank" her is not going to be enjoyable by anyone except you.
You want "psychological horror"?
Have someone call her and your family pretending to be the police department and say that you passed in a vehicle accident.
Point a gun (fake or not) at her and pull the trigger.
Find some roadkill and put it in her mailbox, then car, then bed.
Have some friends stalk her at her work or school and then follow her home.
Lay on the floor with an empty bottle of sleeping pills and a note that says "I'm sorry".