There's been a lot of uncertainty about the early war IJN aircraft color over the years. I learned something about it back in my plastic modeling days - alas, I didn't learn it before I painted a bunch of models in a very wrong color...
Decades ago, when I first began modeling, the color was described as a light gray. Some said was quite similar to the gray used on US Navy planes that would later be called Light Gull Gray. Others said it was lighter, almost an off white. A few claimed it had a slight greenish cast.
The latest and most authoritative word seems to be Ian Baker's Aviation History Coloring Books number 36 & 37, Imperial Japanese Navy Aircraft Colour Schemes, Camouflage & Markings 1937 - 1945, Volumes 1 & 2. According to Baker, the directive that introduced the color we usually call "gray" specified that the planes be painted "dead grass color" which he describes as "medium to light brown or ocher."
I'm not clear about whether that color was retained for the under surfaces when the dark green upper surface color was introduced. It seems likely, but I don't know and Baker isn't clear on that point; or at least, I saw no such clarity in the quick perusal that I had time for this morning.
Baker's books came out back in 1999, but just last week I read something that confirmed his interpretation. In the book First Blue, which is the biography of Roy "Butch" Voris, the WW2 ace who later formed and led the Blue Angels, he and his fellow pilots described the Japanese aircraft they encountered in the first year or two of the war as "brown."
Based on the comments of Baker and Voris, if I was to paint an early WW2 Japanese Navy plane, I would use something like a light to medium tan or beige.
I haven't seen any kind of code, in any of the common color coding systems, to describe the color, and surely the bright Pacific sun made for plenty of variation anyway. Something like the color of the cork in a wine bottle might not be too far off, or maybe something just a tad browner than that. Sort of a "dead grass color."
Anyway, the idea of light gray Japanese Navy planes seems to have gone the way of purple Rufes - nice looking in paintings and profiles, but not representative of the real thing.