Four tail ribbons are generally used:
1. Red. A red ribbon means the horse its attached to can't be trusted - the horse is likely to kick or bite. It warns other competitors and riders not to "tailgate" the offending animal or otherwise get too close.
2. Green. I mentioned green ribbons when talking about green horses. A green ribbon is sometimes put on a young animal or one that hasn't been to many shows. (Regardless of age, I would always put a green ribbon on a horse that had not shown before). Green horses are less predictable than well-trained ones.
The other two colors are seen less often:
1. Blue. A blue ribbon is traditionally put on a stallion. Stallions can be harder to handle than mares or geldings and may, again, do something a bit unpredictable (or very predictable if you know just how testosterone-fueled stallions are).
2. White. A white ribbon is sometimes put on a horse that is available for sale or lease.
I've also heard of pink ribbons for a mare in heat, but these days that's just as likely to mean the rider supports a breast cancer charity...