Bob- yes I`ve seen those here too in similar places, and amusement arcades, fairgorounds, that sort of thing. Actually they are probably complete light fixtures not just covers. Fairground lighting is a little bit of a mystery but I remember seeing some for sale a while ago. It used small 50-60 volt bulbs (presumably wired in series to the mains, but could have been from a transformer or generator) and sockets with snap together coloured covers.
I think it done this way cos it is cheaper to maintain- the little clear bulbs come dirt cheap when bought in bulk, unlike coloured bulbs which would be bought in smaller quantities of each colour and cost more to begin with. Low voltage bulbs ar eusually a little more robust too and hold up to vibrations during transport better than mains bulbs do.
btw sorry all my pics are broken right now

, I`m working on a solution but since I`ve posted so many pics both here and on other boards I`m a member of, it`ll take time to find and fix them all once my solution is in place. If anyone wants to saee any of my pics, email me and I`ll see if I still have them here.