There are 'green mantles', which seem to stand for virginity in quite a few songs. (Off the top of my head I can only think of a version of Tamlin and 'her mantle so green'.) Possible link? We're still in the realm of green things at least. And virginity has been suggested as what the bunch of thyme symbolises in 'the bunch of thyme'. (Someone else once told me it was probably a recipe for soup, but there you are.) Could just be a green buds/shoots young vs rest of the plant old kind of analogy taken a bit further.

Having said that green is robin hood's colour as well and he's quite a virile character in english folklore isn't he. (I'm thinking dodgy may-day celebrations and stuff.)