You were dying the hair of Robin, your boyfriend. He let you dye his hair for once since you always begged him to. And he wanted to dye his hair a blond. And you had the fun idea to die a streak of his hair pink and you did it without hin knowing it. You helped him wash his hair afterwards and dry it. As he then looks at his dry hair in the mirror he's quite pleased with the blond before spotting the pink streak. He sighs slightly and look at you Did you really need to do that?