The following, from Newton Castle is the most detailed account I can find on the net:
"In love with a man who finally turns to another woman, Lady Jean tried everything to have him back (getting new and very nice clothes, shoes with silver buckles to be more attractive...) and started to sing love songs below the castle, but nothing worked. So she went to a witch, who told her to dress in the witchin' claith o' green (getting grass, branch and bind them with a reed) then take them, go have a sit near the Corbie Stone and wait. That is what she did.
After a long wait, she started to hear laughs and felt something pulling her clothes. She finally fell asleep. When she woke up, in the morning, she was dressed all in green.
Everything happened like the witch said. Lady Jean had her lover back and they decided to get married very soon.
She could not get her green clothes off and her future husband was intrigued. As soon as the wedding ceremony started, the groom knew something was wrong with the woman. When he took her hand, he noticed that it was as cold as death. At this moment, Jean cried awfully, and fell in the floor, dead. Her body was laid on the nuptial bed.
She was buried near the castle. The legend says that her gravestone is said to turn on itself three times every Halloween night and that her sad songs can still be heard in the Newton Tower.
The phrase witchin' claith o' green is used on several sites, and is presumably quoted from the common source of the anecdote, which unfortunately few of the site compilers concerned have had the elementary courtesy to acknowledge. The site quoted above does, however, refer to a book called Scottish Witches. No author is named, but it would appear to be Charles W. Cameron (Jarrold Publishing. 1990. ISBN: 0711704511; apparantly out of print.) Cameron was a professional illusionist and writer of popular "occult" books; writers of that kind frequently fail to identify their sources, too, but you never know your luck. So far, like Ian, I'm inclined to be sceptical.