Perhaps someone can tell us how closely Wiccan practices conform to traditional beliefs, how consistent those beliefs were, and how much is accurately known of them. It certainly is interesting how significant colours and plants were in tradition, and how those beliefs survived in various forms into the the modern era. The Victorians, for instance, had a complicated etiquette around the meanings of flowers, which I suppose must have derived from folk beliefs, and they certainly associated the colour red with sexual promiscuity.

Green was also connected with sexuality as Marje says - in Tam Lin Janet, "kilted her green kirtle a little abune her knee", and then went off to (and with), her hero. However, I think the significance of green was less to do with hiding grass stains and more to do with its being the colour of new growth, and by extension of reproduction.

Having said that, Bessie Bell's and Mary Gray's choice of green shoes is still a mystery as the ballad seems to have nothing to do with sex. The two women built themselves a shelter in the woods to escape the plague, but they were infected anyway and died. There is however a traditional back story, which says that a young man brought them food and in doing so he also brought the plague to their hide-out. Perhaps he brought more than that.