Answer:
People who participate in a game known as "snapping". The Snapper snaps the others Jelly bracelet and gets a "Reward".
Yellow- HUG
Red- Lap dance
Black- Normal sex
Purple- three some or Kiss
Pink- Hickey/Flash
Orange- Kiss
Blue- Oral sex on a guy or anal sex
Green- Oral sex on a girl
Clear/Glittery- "What ever"
White- Flash or "What ever"
There are variations to some but these are most common.
People wearing these dose not mean they participate.