I also had the paint pellet idea, but I really kinda sluffed it off because it’s fraught with problems. You’re going to need to use permanent ink, and it’s probably going to need to stain both cloths and especially skin if you want to be able to identify them later. But people who get marked will just be able to discard the clothes and cover skin that is marked.
Then my darker side came up with an idea to use an aerosol spray that makes people sick enough to need to visit an emergency room with very specific symptoms but not be life threatening.
But there are two problems with this. First is liability, as there are inevitably going to be people who either claim or actually do experience more lasting complications from it due to other medical conditions. Second is the fact that these people seem to have a much higher level of organization than the media will let on, and I believe they would adapt with counter measures, such as their own triage centers and even medications that counter any temporary sicknesses.
I like your deeper level of analysis.
And your appreciation of the skill level of the enemy.