Working boat sails were dressed with this wierd and noxious mixture known as "cutch" which in the west usually involved a mix of oil, red oxide powder and boiled oak bark, for the tannin. Several recipes abound according to local tradition. Try googling for Cutch and see what you get.
Always a nasty substance, never dries but rubs off, has to be repeated regularly. In days of yore when I was a sailmakin', I got one enquiry in 5 years - pointed them towards dark red Dacron - far more sensible all round.
You can also get a fake canvas terylene cloth which looks like canvas and stretches as much too. Duradon name, but all these are self colouring and don't need cutching.
Years ago my dad ordered his Halcyon 27 with tan sails as they are easier to look at and therefore trim on a sunny day. The current owner has replaced them with tan sails as well.
I think they look nice and homely and workmanlike on an ex-working boat or something small and unpretentious. My 1880's cutter had what looked like the original sails - at least they were cotton, rotting, and left red stain everywhere. Ideally one needed old overalls and rubber gloves to handle them.
They also look nice I think in combination with white - eg tan for the main and staysail, white for the jib and topsail.