Of all the feral children stories, the green children of Woolpit seem the most curious. They were two Flemish children who walked up to farmers in Suffolk, England, in the 12th century. The children, a boy and a girl, both had green skin. After dumping a fanciful story of a distant twilight land called "Saint Martin", which may or may not have been true, the children were adopted into the community and eventually went on to live normal lives and regain normal skin color.
It turns out that the skin pigment could have been a symptom of a nutritional deficiency, called 'hypochromic anemia.' Similar to how leaves turn color in the fall, the lack of red blood cell pigmentation simply leaves other elements of the body to lend a skin color instead.
Whatever you do, do not search Google images for 'hypochromic anemia'. They're not nearly as pretty as you're picturing it.
But perhaps encounters with people afflicted with this condition accounts for widespread folklore tales of little green elves, gnomes, leprechauns, and other mythical humanoids - maybe they were just malnourished, and so short, and anemic.
