My daughter make a cloak at a Live Action Role Playing Props workshop a couple of years ago. I found some sort of thin faux suede that was a grey-green. It was great because it would 'change color' as it moved in the light. You'd even have time to make it because the cloaks don't take long at all, even if you put a hood on it. I've made one in an afternoon for one of my older sons.
Anyway, with a long green sash and some type of weapon it would be a great costume. If I dressed up, ever, that's what I'd wear.