I forget the brand, but one brand actually has a color labeled "sail." While sails could be any color, typical sails were linen, a sort of off-white. I have used "sand" colors. A red, almost rust color was also popular, apparently from some preservative that sails were dumped in and then dried. US ships often used white cotton sails because we grew so much cotton, and it was cheap here, but white sails were rarer on non-US ships.