The rules for Saint Patricks day when I was a kid were as follows:
If someone isn't wearing green, you get to pinch them (only once for the day)
If you pinch someone who is wearing green, they get you pinch you back.
If they were wearing green but it wasn't visible, (like green underwear) they don't get to pinch you back.
I just used a blue dry-erase marker and a yellow highlighter to make a green mark on my hand to protect myself from pinches.