"Greenlighting is when a male or female (often bisexual) will wear a green shirt, either polo or otherwise, and "pop" or pull their collar up. This marks them as being ready for sex with anyone who chooses them, be it male or female, they are "collared" when someone approaches them and pulls their collar down. This leads to sexual intercourse in most cases or sodomy when both partners are male. The trend is thought to have started in the gay community but quickly spread throught out 2004, with more communities meeting up in real life, and sharing stories online, the term can also be found online on the "Urban Dictionary". Many women who "collar" males see it as a chance for no strings attached sex whilst guys enjoy it for obvious reasons, in gay circles it is seen as a way of openly showing your true feelings. green Lighting shows that you are open to anything and many worry that this may lead to horrific abuse, and whilst this has yet to occur it may only be a matter of time."
Quoted from www.greenlighter.org FAQ.
Hey man, I think I can see a fine ass greenlighter on the other side of the club, I think I'll go collar her.