I just unearthed a rather wealthy stash of semi-rare Great Kanohi masks. Never knew I collected them! Anyway, among them was this clear transparent Kaukau (Great Mask of Water Breathing):
You may recognize this mask in transparent blue as worn by Toa Gali and some Ga-Matoran (notably Hahli) on Mata Nui.
But what stumps me about this mask is the fact that it's not blue; it's colorless. I've looked in Peeron but it doesn't list this as one of the colors it occurred in.
Does anyone know which BIONICLE set this mask may have come from?