Make it a VERY light gray... you'd be surprised how warm it gets underfoot if you go with even a medium gray. The original R28 nonskid was a very pale green colour that seemed to go OK with most other hull colours. Too-dark deck colours can get impossible to walk on barefoot!
If the original deck surface has been "done in" then you are essentially starting over. Once you get the surface fair then you can start again. I would suggest painting the smooth deck areas first, then taping off and putting the non skid on top of that. This will involve a little less taping. If you pencil off the to-be-non-skid areas you needn't paint
those areas fully on the first go around. Then tape off your non skid pattern and apply that.
If you're having the non skid sprayed, add Awlgrip
's "Grip-tex" or similar material, and throw a ball bearing or two into the paint
pot to help keep things stirred up, you also need a very large bore paint
nozzle to avoid pluggage. This is generally done in one heavy coat (careful, lots of paint
will come out with the large nozzle)
Alternatively if you roll it out, the Interlux Interdeck leaves a great gripping surface - better than sprayed - but durability may be an issue (it's a single part paint). I have a my cockpit sole done in Interdeck, and 2 seasons in it's still great.