Badger's sails were made of proofed cotton, but I think most of the proofing had long since evaporated - the water only 'beaded' on them for a month or so - and they were never covered. They didn't sit around wet long enough to get mildewed, because we sailed the boat a lot. Making them was perfectly straightforward and they didn't appear to shrink. The fact that they were secured along the boom, yard and battens may have had something to do with that, or maybe it was because it was pretty heavy cotton, or maybe it was because they were initially waterproof and stabilised before the residual proofing disappeared. They lasted for a Scottish summer, followed by a voyage to the Canaries, Windies, USA and back to the British Virgin Is, by which time they were pretty rotten. So they were about 2 years old.
What do you mean by green? It can take more than 20,000 litres of water to produce 1kg of cotton; equivalent to a single T-shirt and pair of jeans, with most of it grown on irrigated land. It accounts for 24% of the sale of insecticide and 11% of the sales of pesticides, world wide, while accounting for only 2.4% of the world's cropland. GM cotton uses less of all three.
If you found cheap cotton that had been sitting on a shelf somewhere for many years, like we did, you would probably use up something that otherwise would end up being binned. Buying new, commercially grown cotton cloth, as distinct from a polyester, acrylic, etc, fabric, is a complicated ethical cost-benefit analysis that defeats me! That being so, I decided it was better to go for something man made that should last about a decade, with luck.