go for the clear plastic.
most gh films/plastics are 6ml clear plastic.
these films are rated to last 4 years, after the first season they become cloudy, kinda white.. the more sun they get, the cloudier they get
you could of saw a pic of a weathered gh.

nursuries do use white plastic. this is mainly used to overwinter perenials, shrubs and trees. nusuries will also paint their gh white in the summer months to shade them and prevent heat build up.

hope this helps... happy growing