Mike - I saw something over on j-aircraft.com about NMF aircraft. Prior to the attack many of them were NMF, by the time of the attack they were all painted. The actual color they were during the attack is a very heated subject among people much more knowledgable on the subject than me. I recommend checking out www.j-aircraft.com and see what you come up with.
The grey-green joe mentioned is one notion of the color in the first months of the war and that the camramel coloring was due to oxydation, the other side of the camp believes it to be amiero which is the caramel color, they have actually recovered a Zero that took part in the attack and shot down that has paint slotches on it, it was the caramel color but basically it started the whole oxydation/original paint argument going again versus solving the question.
One thing that both sides agree on is that the grey green WAS used in the war, but on which planes and did it weather to the caramel there is no answer yet.
Here are some paint chips to help you match a color no matter WHICH one you decide to use.
http://www.j-aircraft.com/research/gregspringer/wem-review/wem-review.htm
EDIT: pay close attention to his comments on the colors in the grey-green, amiero section, he has examined many relics and gives some dates for which part of the war the paints were used in.