Any chem majors in here?:mellow:
I was a chem minor, but I got mine from a biology major during the prank war when I put it in someone's Pepsi Blue.
There is a food coloring sometimes sold as 'Natural Red' that will turn urine red if you eat enough, but I would not suggest using it as a prank. I know far too many people who would run to the ER if they thought they had blood in their urine, and that is an awful expensive prank if they don't have insurance.