Japanese researcher Katsushi Owaki made the following illuminating comment at the Fuku BBS forum (translated):-
This is something that I posted on j-aircraft.com several years ago.
I polished the exterior of a piece of a Zero model 22 (Mitsubishi serial #3353) that I happened to have with a cloth. As a result, an inner paint that is shown on the bottom left appeared. From my past experience, I have no doubt that the color that appeared was 'I3' (the 3 indicated as a subset) of IJN Kariki 117. Of course, the top coat gray is still there.
It hasn't been so long since 'Ameiro' disappeared in comments abroad, and I'm afraid that there will be another uproar saying, "The Zero's color is I3!!!"
Researcher James F Lansdale maintains that the grey top coat is a matt "chalking" caused by exposure/weathering and may be rubbed through to reveal the original exterior colour of I3. Some of the artifacts in his collection which are said not to be weathered show an original glossy colour similar to the appearance of the I3 chip in the Kariki 117 document.
A difficulty arises because the nearest standard Munsell value for these artifacts, Munsell 5 Y 5/2, is actually very close to the nearest standard Munsell value for J3, Munsell 7.5 Y 5/2. The DE2000 difference between them is only 1.71 (2.0 or less = a close match). However when one compares the very precise fractional Munsell values of the chips from the original colour standards Kariki 117 and the 1945 8609 document the differences are much more pronounced with measurements of 10.7 (early J3 to I3) and 7.76 (late J3/2-6 to I3) respectively.
Given the known effects of age darkening and ambering of the original paint it is perhaps misleading to insist that the current appearance of the relics evidences a topcoat of I3 'Tsuchi iro' (earth or clay colour) rather than J3 'Hai iro' (ash colour), especially as the Zero technical manual refers to 'Hai nezumi iro' (grey rat colour) and the YoKu Report No.0266 of February 1942 describes the paint colour of current Zeros as "J3 (grey) (but) leaning slightly towards ameiro (amber or candy colour)". In other words the colour was a slightly brownish or warm grey. J3 changed to a warmer colour over time and the effects of age on the paint have exacerbated the similarity between the colours.
J3 has a little more green and may have originally been more grey in appearance. The use of plant resins as binder may have made the colour more amber over time and this appearance has been increased with age.