In Tam Lin, Janet is also "green as any glass," which I take to be both a symbolic reference to new growth, etc. and also a literal reference to her having morning sickness from being pregnant. An article "Folklore and Symbolism of Green," by John Hutchings discusses, in part, the fact that green was considered bad luck in British folk belief. One informant he writes about talked about the belief that black (mourning) would follow green. This is related to the leaves on the trees dying. In the book Morning Dew and Roses, Barre Toelken writes that "virtually every time someone gets dressed up in green in a ballad that person is dead by the end of the story," though I can definitely think of exceptions to this rule. So, I'm not super familiar with the particular ballad in question with this thread, but as the heroines appear to end up dead by the end of the story, that might relate to the concept of black after green, etc.