Yup - red for horses that might kick (or, in our case, are bothered by other horses too close to their back end). Green ribbon shows that it's a green horse and as such might behave unpredictably so is again really warning you to give it plenty of room.
White is for stallions too. Its annoying how many people don't know this (or don't bother to find out) but I put them in so if someone does decide to use whatever horse I'm on as a bumper and it kicks or whatever I have a come back!
Shame people didnt take more notice of red ribbons, we always put a red ribbon in our mares tail, not because she is a kicker, she has a habit of leaping in the air if too many horses crowd her, just in excitement and does tend to buck, never actually kicks out but I feel safer knowing I am warning others to keep their distance but noone ever seems to take any notice then if she does ever do anything I feel at least thay had been warned. We try and keep her away from groups of horses and only bring her near when she is walking in to ring.