When I started working at sea, red lead was a basic metal primer paint which often never went of hard, but made surface over itself much like boiled linseed oil which at one time was a favourite coating for steel decks, as when it was scored it just remade a new surface and I have seen ships go for scrap with perfect decks because they were only coated in boiled oil. Have been told by an old tankerman where he knew of some old tankers where old engine oil mixed with diesel was used as deck coatings also with no paint covering.
Also I suspect old time ship owners probably bought black paint as that was what was available and cheapest and also suspect early vessels were tarred before paint came available and ship owners were traditional in outlook. Dutch Coasters and barges until recent years used coal tar on their ships sides, I have seen it used by them in my youth and a old master swore it was the best stuff to treat steel sides.
As for red antifoul now, besides traditional views, it could be due to the chemical make up as you could get green antifoul once but that was stopped because it contained copper which made it illegal under environmental laws. When I saw the liner Orsovia in Tilbury drydock, the lower part of the hull near the bottom was left bare steel just before they re floated her.