I use lime strip in the appropriate width for all the deck planking I do. Maybe I'm wrong but I think to scale the caulking would be too small, so I just run a black fairly fine tip pen down the seams.
On big real ships, planked decks were never painted, always scrubbed. In the Royal Navy it was one of the first jobs in the morning to do that. On small ships, like my beloved Thames Barges, decks were painted, normally a palish blue and most skippers would keep a supply of paint on board to keep them looking smart.