I believe in an old thread it was said that the static structural lines i.e. those that were there to hold masts etc and were never changed or climbed were tarred therefore black.
Those that were climbed or adjusted to move yards etc were natural i.e. hemp / brown, as they were affected by the salt water they rotted and got darker even to the point of being black and generally were replaced more frequently, would not expect any rope to be white in normal use, maybe in special ceremonial cases.
Dlcast would be best to advise.
