Elphaba's green skin is explained in the story by her mother drinking a "green elixir" the night she conceived Elphaba. This magical or mysterious green potion, given to her mother by an unnamed man (implied to be the Wizard of Oz), caused Elphaba to be born with green skin. In the musical and movie adaptations of Wicked, it is a key plot point that Elphaba's green skin is a direct result of the green elixir in her mother's system during pregnancy. This green skin sets Elphaba apart and symbolizes her unique heritage and powers, which stem from her parents—one from Oz and one from Earth. The green skin also plays a significant role in how Elphaba is perceived socially and narratively as an outcast and a powerful figure.

The story around the green elixir varies slightly between versions, with the musical showing a consensual affair and the novel hinting at a more sinister encounter involving the elixir being used to drug Elphaba's mother. Nonetheless, the green elixir is consistently the cause of Elphaba's distinctive green skin.

Thus, Elphaba is green because of the magical green elixir her mother consumed, making it a fictional and symbolic element tied to her character's origin and identity.