...isn't it the glass that's green?
Ep 7: Middle Muddle
Stephen Fry stated that mirrors are slightly green, and while I don't doubt that, I'm almost positive it's because of the iron content in most sheet glass. As per this article: http://www.cgcglass.com/lgreen.htm
Stephen was incorrect in saying the glass is colourless as no commercially manufactured glass is truly colourless.