since the show was cancelled, are they going to reschedule it? if they are, then here's what i think:
at first i thought you just meant a red ribbon meaning to make your horse's tail look pretty. if i don't know what a red ribbon is, then there's a good chance that a few others won't either. so, therefore, i would put the red ribbon on your horse's tail to be safe, and tell the other riders in your class to avoid red tails as well. then, everybody wins! everyone is safe, and you gave the other riders that knowledge so they can be safe in future shows!