What's up with our popular conception of witches as a green-skinned hags? It is, after all, one of the most commonly witch-related Google queries.
The simplest answer is Hollywood. Though the 1939 classic The Wizard of Oz was adapted from L. Frank Baum's The Wonderful Wizard of Oz, it, like most film adaptations, wasn't entirely faithful to the original.
The color green features prominently in Baum's children's book, what with Dorothy on the hunt for Emerald City, Toto's green ribbon around his neck, and her run-in with a "little man...clothed all in green, from his head to his feet, and even his skin was of a greenish tint. At his side was a large green box." But the literary Wicked Witch of the West isn't green-skinned, nor are Dorothy's prized shoes ruby red.
The decision to upgrade Dorothy's silver slippers to sparkly red ones was motivated by Technicolor. The Wizard of Oz was one of earlier big-budget Technicolor productions, and producers opted for ruby slippers that would pop against the yellow brick road. There's a decent chance then that they painted Margaret Hamilton's face lizard green (with potentially toxic makeup) for similar Technicolor-related reasons.
Of course, the Wicked Witch of the West might've also gotten her green skin from the color's previous otherworldly associations. There's green absinthe, nicknamed Green Fairy, thought to cause hallucinations. And speaking of hallucinations, alleged witches in the Middle Ages were thought to get high on hallucinogenic ointments said to have had greenish tints as well. But regardless of whether the MGM makeup department was informed on color symbolism, the green-skinned Wicked Witch of the West was the right cinematic call. Just ask Google.
Related reading: The Mother of Modern Witchcraft