I wouldn't pull a prank on my poor horse like that. I'd flat out go to the people I suspected. No way I would risk my horse getting hurt.
You can just take a hose to kool-aid, no harm done :) Dye, on the other hand, would be dangerous. But we are just talking harmless koolaid. Much like dying your own hair with it. It washes out :)
Sounds like a fun summer camp prank... get some blue koolaid on a gray pony the night before the anual bareback race...