Same as the deck, from what I can gather. The deck was wood treated with carblinoulieoumbumdum (spell), and tho there seem to have been 2 types, a common was a charcoal col that matched the hull paint and the anti-fouling paint. The difference would be slight, between the deck an the metal, you might make the metal more solid charcoal, and the deck more thin and washy. The deck grates were wood, but there are some metal hatches. Dogie's Uboat colours article (AMP website, Library) will have more indepth info.
