When I talked to my brother about my plans to write about witches, he said, among other things, that maybe I could explain why some witches are green. I told him I had thought about that, and had not reached any completely satisfactory answer. Here's what I've come up with in my search so far.
A quick Google found one site that asked the question, took answers from a group of more or less anonymous people, and then voted on what they thought was the best answer. (A less likely way of reaching a true answer is hard to conceive.) The most popular answers bore all the earmarks of folk explanations spun out of the airiest speculations. One said it was the memory of the bruised faces of accused witches about to be executed in the old days; they said this was the one time most ordinary people saw accused witches. Another claimed it was because witches had ties to fairies, that green was a fairy color, and also tried to get the Green Man into the pagan mix.
The trouble with these theories is that there is very little evidence of green witches before the first half of the 20th century. Most depictions of witches in writing or illustration show them as no different in coloration from the people around them. The earliest evidence I can find of green witches is a nursery rhyme of unknown age and provenance, but possibly Scottish and almost certainly before the last century:
"This is the night of Hallowe'en
When all the witches might be seen;
Some of them black, some of them green,
Some of them like a turkey bean."
How serious this description might be and how much it was used for the rhyme scheme may be deduced by the comparison to a turkey bean.
The least popular answer, but the most likely explanation, was that green witches had their origin in theatrical and even cinematic origins. There seem to be very few green witches before Margaret Hamilton portrayed the Wicked Witch of the West in The Wizard of Oz; after that impressive and even traumatizing performance, green skin on a witch seems to have become de rigeur. In the theater, sickly shades of grey, green and blue had long been used to signify supernatural creatures by giving them a fearsome, corpse-like hue. In the new eye-popping world of color cinematography the matching of vivid green and inky black made a distinctively menacing combination.
But the mention of this livid coloration starts another hare of argument. There is a supernatural witch, the Cailleach Bheur, a blue-faced lean hag who personifies winter. There are widespread traces of her throughout England, and in Scotland and Ireland and the Isle of Man. She was reborn every All Hallows and went about smiting the earth with cold and snow, then cast down her staff on May Eve and either turned into a gray stone or a beautiful maiden. There are many tales of her under many names; she is the Loathly Hag in Chaucer and 'the blew, meager hag' in Milton.
In the color sense of older times, blue and green are very close. Could the green witch of our time somehow be a distant descendant, from folktales to early theater to movies, of the blue-faced Cailleach Bheur? It is, at this time, almost as tenuous a speculation as those I have cited before, but one perhaps worthy of further research.
Or perhaps it's all as simple as my nephew says: "Witches are green because they drink pickle juice!"