Colored Water as an Identifier

In a number of countries police are using colored water shot from water cannons as a means to identify protesters. In some cases the water can cling to one's skin for an extended period of time, thus protesters can be identified long after the protest is over.

Of course, the water cannon does not discriminate between protesters and innocent observers. But, hey, it's the 21st century and we're moving to a world that doesn't want to spend much time separating the guilty from the innocent.