The Pinkan Berry is a berry found on Pinkan Island of the Orange Islands. When Pokémon eat the berries, they'll turn pink.
Ash's Pikachu ate some of the berries on the island when he, Misty and Tracey landed there by accident after getting washed on the beach by a series of whirlpools. At first, Ash got worried that Pikachu would stay like forever, but Officer Jenny, who ran the Pokémon reserve on Pinkan Island, assured him that it would only be temporary for his Pikachu.
Jenny went on to explain that Pokémon only stay pink if they eat the berries all the time, which is obviously true for the Pokémon who live on the island.
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