I think I asked the same question a couple of years back. If I am remembering correctly there were directives for painting USN vehicles upon land but nothing said about USN vehicles aboard ship.

So I am going to guess and say the same color as flight deck stain or as close as possible. If they were changing aircraft colors and then flight deck stains to somewhat match to aid in deception of air group numbers and targeting I would think having tilly, jeeps and BNO-40s running in Navy Blue Gray would stand out. So I would guess the same color as flight deck. Maybe a shade or two in differences possibly .

There is a picture of tilly and crash crew resting beside an Essex class island. I believe it was Lexington. If so the island was 5N. Tilly was not much different.

Best guessing on my part but that is how I paint my Karry Cranes and BNO-40s etc. just slightly lighter..