This hypothesis takes the current appearance of the Iida Zero artifacts and projects the effect of reducing the age darkening in stages towards the faded and bleached example of the Niihau artifact.
The stages of 25%, 50% and 75% have been calculated. Most interestingly in all cases the closest matches continue as FS 16350 and RAL 7034 Gelbgrau (yellow grey) with the difference calculations shown in parentheses. In the case of 25% reduction the closest Munsell is 7.5 Y 6/2 @ 4.03. In the other two cases the closest Munsell is 10 Y 6/2 @ 2.65 and 1.24 respectively.
Comparing the current measured value of the Iida artifact against the later J3 and I3 values results in DE2000 difference calculations of 4.32 and 7.79 respectively. The reduction values compare as follows:-
25% to J3 @ 4.12 & to I3 @ 9.04
50% to J3 @ 4.41 & to I3 @ 10.30
75% to J3 @ 5.08 & to I3 @ 11.60
The Niihau example, assuming a fade of 100% from the Iida example, measures against the J3 and I3 samples as 7.31 and 13.40.
In all cases the actual and projected artifact values are closer to the J3 sample by less than half their difference to the I3 sample. Assuming the values of these two samples are correct, it is improbable that the original paint applied to the Iida and Niihau Zero artifacts matched I3 but more probable that it matched J3. We might reasonably expect the original paint to have been slightly lighter and "greyer" with less yellowing (amber), which in turn will decrease the green caste. But please bear in mind the contemporaneous description "J3 grey colour (but) slightly towards amber colour", which suggests the original paint was a warm grey with a slightly yellow or amber caste to it.
Image credit: Rendered chips ©Straggler 2008