It's been a while since I've read about it, but I believe the consensus was the armored glass in the cupola was generally clear, usually tinted green for scale effect in modeling. I think the dark tinted versions stem from modelers painting them that way because the part was molded solid.

I don't know about modern armor with electronics or optics, etc, but in WWII, I don't see an advantage to a dark tint of a glass you need to see through in a whole host of conditions, including night.