From Bulbapedia, the community-driven Pokémon encyclopedia.
Rainbow Valley (Japanese: 虹の谷 Rainbow Valley) is an anime-exclusive location in the Unova region. It exists somewhere along Route 4.
Rainbow Valley is a place where Cottonee gather after finding their partner. They ride on what is known as the "Diamond Breeze" while holding hands in order to get there. The Diamond Breeze will only blow after a full moon. The mountain sides glitter in different colors because of the different minerals they are composed of.
Ash and friends had the chance to visit Rainbow Valley after helping a Cottonee win over a love interest. There, they said goodbye to their new friend.