Every March in Illinois, a stretch of the Chicago River -- between Columbus Drive and Wabash Avenue -- is dyed green by St. Patrick's Day Parade Organizers. It's fun. It's festive. People come from all over the world to see the normally murky blue waters turn… well, murky green. It's a spectacle. According to the Chicago Tribune, the recipe for the dye has been a well-guarded secret for years. "It's our secret, and we intend to keep it that way," parade committee Chairman James Coyne said.
https://www.thrillist.com/drink/nation/how-to-make-green-beer-st-patricks-day