We do checker boards, hearts, things to match their names, etc. It is kind of like wearing a vest instead of a heavy coat. This way they don't freeze on the off days of work. We use chalk or tape to lay out our design before clipping. If it is cold, damp or misty, please make sure your horse has either a water repellant sheet or blanket, or a run in shed.
Horses are tough, but once they get wet with cold, they can experience hypothermia.
Also if your horse is clipped, he definitely will need a sheet or blanket to replace Mother Natures fur coat that we took off. The last thing is don't forget to keep salt blocks with your water troughs. Just because it is colder, doesn't mean they don't need salt. Also if your water troughs are freezing, go to your local feed store and order a water heater that is used for water troughs. They make them for cattle and they do not shock your livestock while in the water.