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When said grey all of you have to consider chemical process which due to the time change color. I know color sample of WW2 color [mean not only the age but also the quality] which was in original ocher but during the time changed into the grey and the sample are keep in home out of sun light.
There should be also taken interaction among the pigments and liquid where it was mixed. The fact that this color rapidly changed and this changes could be noted in weeks have to give high attention when look at the very old samples.
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