For early period black. They were coated in a waterproofing material. Rope was made with hemp in those days. "Hemp fibers had to be soaked in pine tar to resist deterioration in the tough environment at sea."
"The USS Constitution in its recent major refit got synthetic standing
rigging, black of course. Part of the maintenance routine on ships with
hemp was tarring the rigging. Tar dripping out of the rigging on hot
days was one of the less romantic aspects of life before the mast. On
the other hand, the look of black synthetic standing rigging on a
210-year-old ship with the place the Constitution holds, tied up within
view of the ropewalk that for so long made its rigging, is a little
less....."
http://www.fishermensvoice.com/0410TonsOfRope.html
Hope that helps