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Okay, I'm finally gonna go with "nobody fucking knows".
I looked at the AK set, the Hataka set, and the old extinct Gunze set (pic) for IJA tanks, and compared the paint instructions on Scalemates from different Tamiya and Fine Molds kits, and the old Dragon and current 2025 Dragon Chi-Ha kits. All gave wildly different color codes.
Nobody fucking knows the exact shade of these colors. They know the official names the paints were given, but as far as the EXACT color: it seems to be mostly guesstimates.