Chicago River dyed green for St. Patrick's Day celebration. 12 March 2005. Photo by Knowledge Seeker (Own work) [Public domain]. Available from Wikimedia Commons.
Since 1961, the Chicago River has been dyed green for St. Patrick's Day by the Chicago Journeyman Plumbers. Stephen Bailey noticed that a plumber's white coveralls had turned green and asked him how they got that way. It turned out that the dye used to detect leaks turned green.
Source: Dyeing of the River: Modern Day Miracle