This comes in the second half of Orlando’s century-spanning adventure, that is, the female half of his-now-her life. When she returns to England after a long absence (somewhere between 20 and 200 years, you can never quite tell), she discovers a pile of court cases based on her being supposed dead, or discovered female, which (as Woolf puts drily) amounts to the same thing. The court cases drag on for at least 100 years before being finally settled in her favour. The snuff-coloured gown would presumably allow her to watch the latest developments without being recognised as the defendant.
‘And so back again to town, where she would don a snuff-coloured gown like a lawyer’s and visit the courts to hear how her cases were doing, for her fortune was wasting hourly and the suits seemed no nearer consummation than they had been a hundred years ago.’
Source: Virginia Woolf, Orlando: A Biography, ed. with an introduction by Rachel Bowlby (Oxford: World’s Classics, 1992), p. 212