You must have seen photos of protesters being doused with colored-water cannons by the police. Using water canon is understood as it’s an easy non-hazardous way to disperse mob, but why would police spray protesters with purple and pink water? Simple: 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.
The most famous use of colored-water cannons took place in South Africa in 1989, when police soaked anti-apartheid activists with purple water. But in the ensuing chaos, one of the protester turned a water cannon back at police and towards 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.
During the last 15 years, protesters in Hungary, Indonesia, Argentina, Malaysia, India and Israel have all been showered with colored water. In Uganda last year pink dye was employed to humiliate protesters. In Israel, Palestinian rioters were sprayed deep blue, the colour of the Israeli flag. The Hungarian police use green, the Koreans orange. Indian police is particularly fond of purple.