Why is Green Martyrdom green?

I'd heard of green martyrdom a few times on the radio, but never gave it much thought > Three kinds of martyrdom that are counted as a cross to man, i.e. white martyrdom, green martyrdom, and red martyrdom.—The Cambray Homily (Thesaurus Palæohibernicus, II., p. 246). Green martyrdom somewhat anachronistically seems to have been defined as having something to do with dollar bills being of a greenish hue. I don't think that would be the case in the Ireland when the Cambray Homily was transcribed. So why green? Red and White I understand, but green usually has a different significance than dying.