A photo on the front page of Wednesday's Wall Street Journal showed protesters being doused with colored-water cannons in the Indian city of Srinagar. Why would police spray protesters with purple water?
To identify and arrest them later. Many water cannons on the market today come with a tank specially designed to store a semi-permanent colored dye. If police decide they want to "tag" protesters with the dye, they can press a button to inject it into the main water stream. Once the water cannon is trained on a crowd, anyone hit by the spray will be easily recognizable by police. India isn't the only country to use dye in its water cannons: During the last 15 years, protesters in Hungary, Indonesia, and Israel have all been showered with colored water.
Police say the dye they use has helped them identify rioters in Srinagar, where authorities arrested at least 30 state government employees who were protesting for higher pay. But according to the local daily Rising Kashmir, the purple water cannons also hit shopkeepers, auto-rickshaw drivers, and the Getty Images photographer who took the shot that accompanies this article. Locals have also complained that the purple dye is toxic, potentially causing skin rashes and cancer. (The purple dye is reportedly the same chemical that Indians have been cautioned not to use during Holi, the Hindu color festival.) Srinagar police denied the charges, responding that the dye was "harmless."
Perhaps the most famous use of colored-water cannons showed how badly the tactic can backfire. In September 1989, a group of anti-apartheid protesters marching on the Parliament in Cape Town, South Africa, were met with orders to disperse, and then soaked with a dark purple spray. But in the ensuing chaos, one of the marchers wrested control of the water cannon and redirected it toward the local headquarters of the ruling National Party. The headquarters, along with the historic (and white-painted) Old Town House, were doused with purple. The next day, a graffiti artist tagged the Old Town House with the phrase "The Purple Shall Govern," which soon became an anti-apartheid slogan. But despite the legacy of the so-called "Purple Rain" protests, colored-water cannons may reappear in South Africa: Police have announced that they have colored-water cannons at the ready in case hooligans get out of control at the 2010 World Cup.
In the United States, water cannons are no longer used widely for crowd control—mostly, experts say, because they are still associated with the brutal suppression of civil rights protesters in the 1960s. But law enforcement agencies have still experimented with other means of tagging rioters to mark them for future arrest. For example, the FN303—the "less-lethal" gun that caused the accidental death of an Emerson College student during World Series celebrations in 2004—can be armed with pellets that include paint instead of a more typical pepper spray-based projectile. Since last October, U.S. Border Patrol agents have used FN303s loaded with paint to police fences in Arizona and California.
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Explainer thanks Adrian J. Canton of Adrian J. Canton & Co. and Andrew Mazzara of the Penn State Institute of Non-Lethal Defense Technologies.