"Tanbark" was the predominant color of European working sail before engines took over. The term refers to the bark-based tannins used to help preserve cotton sails back in the day. This "tanning" process helped prevent rot and UV degradation.
If you poke around on the web, the consensus for the old-time tanbark sail recipe was something like "catechutannic acid, vegetable tannins, fermented urine, and fermented dung." I am not kidding.
Traditional smallcraft still use wine-colored sails, though nowadays it's just dyed dacron. While mostly an affectation in the 21st century, red sails are more visible, don't reflect sunlight into the skipper's eyes, and don't show dirt and wear like white sails do.
I chose tanbark for PocketShip #1 for two reasons: to stand out in photographs, and because PocketShip vaguely resembles the gaff-headed British pilot cutters that had tanbark sails.
This photo's for Peter: