‘Forerunner’ originally meant those who ran ahead of a royal progress to mark with white chalk the doors of houses to be requisitioned for the royal entourage.  In his poem ‘The Forerunners’, George Herbert (1593-1633) uses this meaning as a metaphor for the approach of death:

‘The harbingers are come.  See, see their mark;

White is their colour, and behold my head.’  

Source: George Herbert, ‘The Forerunners’, quoted in John Drury, Music at Midnight: The Life and Poetry of George Herbert (London: Penguin Books, 2014), p. 248