Rainbow is the name of a hippie samaritan seen in Muppets from Space.
When Dr. Bunsen Honeydew and Beaker are left at the gas station while Fozzie Bear drives the Muppets to rendez-vous with Gonzo's Family at Cape Doom in the Electric Mayhem Bus, the scientists hitch a ride on a psychedelic wagon with a group of hippies. Although their time is brief, Bunsen and Beaker acquaint themselves with them enough to learn at least one of their names.
Rainbow wears a baja jacket with matching bandana to hold his long hair back, and sports green-tinted shades that rest on the end of his nose. He doesn't speak in his brief appearance, partially due to spending his screen time shoving a handful of Lays potato chips in his mouth.
It's not hinted whether his unique identifier is his given name or an epithet indicative of his wardrobe and mode of transport. Suitably, Bunsen bids farewell to Rainbow, advocating that he "stay groovy!"