When the Wizard bestows Zelena with the shoes, they are silver, as opposed to ruby like in The Wizard of Oz (1939), which is actually accurate to the L. Frank Baum book "The Wonderful Wizard of Oz" that the 1939 film was based on. The shoes were changed to ruby from silver in the 1939 film by MGM chief Louis B. Mayer when he realized that the Technicolor production would benefit.
The title comes from a song from "The Muppets": "It's Not Easy Being Green" sung by Kermit The Frog.