I haven't heavily researched the ACW colors, so I will happily stand corrected by anyone who has.
I expect USN practices were very similar to those of the Royal Navy from the dawn of the Age of Sail until painted steel decks came into vogue. Wood decks tend to look like natural pale wood colors when kept meticulously clean, and darken to deeper browns and grays as they weather or get dirty. Most ships in the ACW were built new for the conflict and many CSN ships didn't survive long, so I'd expect ACW vessel decks to be pretty pale unless stained. A couple other considerations:
- I don't know if there were any regulations or practices for varnishing decks, but if so, deeper browns would be appropriate.
- Coal smoke blackens everything, so it may have darkened the rear deck sections of any steam ship – does anyone know about this?
For pale natural wood I use colors similar to Vallejo Iraqui Sand, though I tend to prefer lighter hues with a bit less yellow.
My favorite deck color paint by far is Old Wood by Howard Hues, though I strongly recommend against that specific paint – it has awful coverage and the jar I bought sealed so badly it dried out in the months between my first and second uses of it. I managed to find a cheap craft paint in a color almost exactly similar in some craft store (Michaels, Hobby Lobby, Joann Fabrics, etc.), plus a bunch of other colors close enough for wargaming.
I recommend buying a selection of such craft acrylics in lighter pale tans and grayish tans, and using them to make slight variations in deck colors between vessels. The actual colors of a squadron of naval vessels would vary based on materials available at the time of construction, the vessel's recent service, the preferences of the captain, the diligence of the officers and crew, etc. It would not be unreasonable to have two identical vessels side by side with different shades of deck planking, and painting them that way really helps the players tell them apart during play.
My favorite wood deck painting technique is to paint the deck surface a color between ivory and pale tan, and stain it with a very very very thin burnt umber FW ink. The ink does a nice job of bringing out deck details, tints the base color with just a bit of brown, and gives an overall very "woody" impression.
- Ix