Black.
The mask is low profile enough, that you would gain nothing in periferal vision even if the skirt was truely clear. In fact, ambient light splashing into the mask from the skirt is actually detrimental with photograqphy and low light conditions. (when your face is pressed against a window you want to look through, you cup your haqnds around your eyes don't you?
Besides people really look like kooks with their facial features all smashed up in a clear skirt; especially with a nice green booger sloping around the nose pocket. Besides, clear skirts age (yellowing) poorly as well.
Just my humble opinion.