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.