If you see pony rides being given (other than on the carousel things you sometimes see at fairs) or little kids being taught English style riding, you might see rainbow reins.
They're rubber covered reins with colored panels - the exact colors vary, but they're generally bright primary colors.
The purpose is to help kids learn to hold the reins in the right place and develop contact. It's a lot easier to tell a kid to keep their hands on the blue bit or the red bit, after all.
They're also kind of cute.