If you're talking modern US flight deck, it can be a tricky question.
I'm sure when it's originally applied, it's pretty close to flat black - however it never stays that way for long. Sun bleaches it, salty sea air also kind of tints it, and then there's airplanes taxiing all over, leaving rubber marks and various fluid stains here and there.
Personally, I'd start with something along the lines of gunship gray to capture a slightly faded black. I'd then randomly apply some darker and lighter shades to capture the look. A couple drops of regular black or india ink could represent small fluid spills (they're celaned up quickly but still stain the nonskid). Salt is most apparent in the tiedowns - you can drybrush that (I'd probably use a very light gray as I suspect straight white would be too stark) or I've also heard of guys using real salt.
Also remember the deck surface is a painted-on nonskid, so depending on scale you might want to try to replicate that texture. However, some companies make aftermarket flight deck sections that show this texture nicely.