When I bought Retreat in 1994, she was 14 yrs old and had a purplish venturi. At the time there were many other boats around with the same purplish coloured venturis, so I didn't think anything was wrong. Yhen I dropped the bimini onto it when I got a shelter and needed to reduce the headroom. I took the broken piece to a glass shop for replacement and was told it had originally been "bronze" coloured, and could only be replaced with bronze or clear. I took the other three pieces in and now, with the bronze venturi about 14 yrs old, the bronze colour hasn't faded a bit.
Mine is curved all the way around. The middle two pieces are each about 4' long, the ends each about 2'. The curve was obtained by heating them and letting them droop until the correct curve was obtained. The glass guy had no trouble getting it right on the first try. I think these are plexiglass, but don't know for sure, as they could be lexan.