they are warnings. something you need to know before riding with other people.
Red is kick warning yellow is stallion green is young and inexperienced white is for sale blue is aggressive gelding and pink is moody mare
Different colors different meanings
red usually means they kick
As the others said. And it's not just for ponies. But colours vary from country to country
To warn riders/make them aware. Red means they kick, green means young horse white is stallion loads more if you google
Google ribbon color chart. They mean different things, for example, red means caution, that pony will kick