This question came up several years back on another forum. Midway and her sisters had steel decks, but I recall one of the answers to this question on that long ago post, included a picture of crew members replacing "tile like" coverings for the deck. Evidently these were non-skid additions for what would otherwise be a very slippery deck. The photo was in black and white so you could not exactly determine the color, but from the photo it appeared like it might be a very dark blue or gray. Were I to bet I would put my money on the former.
Adding to what has been said, I was aboard Kitty Hawk just prior to being commissioned (within days) and her deck was covered with what looked to be a tan colored cement, with a rough pebble texture.
Some years later I was aboard America, while in commission. Her flight deck was covered with something like the Macadam or tar, all black from which the heat of the day reminded one one of Dante's Inferno. Made up my mind there and then that the flight deck crews on those carriers earn every dime they make and more.
My recommendation for a deck color would be a dark blue.
One of my favorite bigger-scale model builds as a youth was the Revell USS Midway.... as I recall, the box art showed her in bare wood decks with chrome-yellow deck markings, a la pre-war USN CV practice. Was there a time (if so, probably between 1946 and 1950) where any of the ships in this class appeared this way? I doubt it, but it would make for a fun re-paint if so!