Witches aren't exactly reliable bogey-ladies anymore – these days, they're less the wicked, warty crones of Grimm's fairy tales and more the pretty, gifted Sabrinas of supernatural YA. Even so, the green-skinned witch is still a potent image and one that shifts a lot of green face-paint and black pointy hats every Halloween. But why green?

The green-skinned crone is actually a relatively new incarnation of the evil witch – in fact, while the evil witch as a cultural narrative dates back millennia, the green skin dates precisely back to 1939 and the MGM film, The Wizard of Oz. Margaret Hamilton's cackling and emerald-tinted portrayal of the Wicked Witch of the West, rendered in vivid Technicolor, is the only reason that anyone associates green skin with witches. As Professor Marion Gibson, associate professor of Renaissance and magical literatures at the University of Exeter and an expert in popular depictions of witches, explained, via email, "There are a few images of witches – for instance, on Halloween postcards – with odd coloured faces (usually red/orange, surprisingly) but MGM's green-faced witch is the first to make a key feature of a completely non-human skin colour."

So the decision to make the Wicked Witch green was not informed by any long-standing green-skinned witch traditions, neither was it inspired by the original Oz books – in L. Frank Baum's 1900 fantasy book, The Wonderful Wizard of Oz, the Witch is ugly, cruel, and afraid of water, but she's not green. It seems that the only reason MGM's famously revolving team of filmmakers, costumers, and screenwriters decided on green was that it looked suitably scary and otherworldly – and that it showed up really well on film. Lavish and massively budgeted, The Wizard of Oz relied cutting-edge visual effects to weave its magic, including the relatively new Technicolor film process that saw Dorothy leave sepia-toned Kansas for candy-colored Oz. A hook-nosed witch with skin the color of a poisoned apple worked, and worked so well that she gave countless children nightmares well into the 1970s – in 1976, Hamilton appeared as the Witch on an episode of Sesame Street, prompting a flurry of letters from angry parents complaining that their children were in tears after the show.

The make-up used to turn Hamilton green didn't only look poisonous, it was: The copper-based paint could have made her seriously ill if ingested, so she was forced to drink her lunches through a straw during the long, four-month shoot. The paint could only be removed with rubbing alcohol and even so, it left her skin tinted green for weeks after shooting. And even worse, it was flammable – during the scene when the Wicked Witch threatens Dorothy and the Munchkins and then disappears in a boom of smoke and fire, Hamilton's costume caught alight. She suffered first and second degree burns to her face and hands and spent six weeks recovering in the hospital.

But The Wizard of Oz and the magic of Technicolor is the reason why witches are green – not, as some have argued, because this was how accused witches at the height of the "burning times", the witch-hunt frenzies of 16th and 17th century Europe that claimed the lives of roughly 60,000 people, were actually seen, especially after they'd been tortured into a confession. As Professor Gibson noted, at the time of the witch trials, the color green was more closely associated with fairies and not battered witches. Witches, in fact, were more often depicted as pale and ghostly, likely a result of their supposed nocturnal habits. As to the other witches' stereotypes – the pointy hats, affinity for cats, flying on brooms – those are all more solidly grounded in European folklore tradition.

The last person to be formally executed for witchcraft was Anna Goeldi, a Swiss maid who may or may not have been putting needles in her employers' food, and was hung in 1782. But that doesn't mean that people have stopped believing in evil beings who consort with the Devil: In America, roughly 21 percent of the population believe that witches – of the supernatural, broom-riding, possibly green-faced persuasion, not the Wiccan – exist, according to a 2005 Gallup survey.