In the film and still photography industry- diffusion filters are used to control and modify light. The filters are designed to be placed between an electric light or the sun and the subject being photographed (they're not the little ones that go over a camera lens for example). These filters, manufactured by a few different companies, are available in sheets or in large rolls. I think in general they're made of polyester, and are not affected by water, rain, or wind. They're pretty strong. Some have woven fibers "spun" into the material. It might be worth looking into this material for outdoor screens. Some are even heat resistant because they're designed to be placed near very hot lights. They are made into hundreds of colors, densities, and weights. Plenty of them remind me of a material found in shoji screens. I'm not sure how well they'd perform long-term, but I'd guess that a number of them would not yellow in the weather.
Two manufacturers I'm familiar with, Rosco Filters
http://www.rosco.com/us/filters/cinegel.asp and Lee Filters
http://www.leefiltersusa.com/lighting/products/finder/ref:C46DD5D575BA37/offer swatch books to make choosing the right material easier. If you have a local professional photo store or professional lighting equipment store they might have one you could look at- or you could buy your own. Keep in mind that these websites cater to proffesionals who generally understand the technical language behind photo/film lighting.
Sometimes finding the right material does not always a straight (or familiar) path. Hope this helps!
-Mark