[Zack The Ripper]

The lights are generally used to signify dead using red, and blue or green as friendly.

Blue/green generally is placed at the back of the helmet so you minimize your "signature" in the dark as generally you will be facing your enemy (unless you are flanked). I personally do not like doing this as even with it at the back of your head it can really give you away in the dark.

Red lights generally go on top, but I have seen them on the sides and back as well. Again, this is usually to signify being dead/out of play.

I have seen the odd CQB game in which one team will wear a certain color light and the other team uses another one it order to help with PID. This doesn't happen often though. The general trend is as described above.

Hope this answers your question.