The Italians, during the riot-a-week campaigns by the Communists some years back, used to put a dye in the water, they spray the crowd, and arrest anyone who wore the "color-of-the-day".
I don't see why some harmless food dye couldn't be done here - non-washable if available. If nothing else, the "hoodies" would be ruined.
Then again, we have the ACLU . . .
Let’s combine the 2 reply suggestions...
water cannon plus blue dyed USED Port-O-Sanfluid equals win.