Thought to have been started in the UK by "chavs". A man will wear a green shirt with popped collar to show he is free for sex with anyone, be it a man or woman. Anyone willing to join him needs only to approach him and pop his collar down, claiming sexual rights to him. Started a few years ago and has spread slowly and is still largely unknown however many parent groups consider it dangerous and force strict clothing rules upon their kids as a result, as well as telling others to do the same!
I saw this kid at the club the other night with a green shirt and his collar popped up. An older dude came up to him and popped his collar back down, and then the two headed off towards the bathroom. They was green lighting.