It's not a theory, it's a fact. I have owned many of the older tractors, and also one post-1990 cyclops and a couple of newer 2000 series tractors, and if you put them next to each other, the difference is like night and day. Some idiot also touched up the fenders on my 1872 with the newer paint, and it looks terrible.
The CCC-built tractors through 1989 used the same shade of yellow as the IH tractors, as far as I can tell. That is what is shown in the chart. All of the yellow numbers in the chart should be the correct yellow for the older tractors; I know the PPG one is because I have used it. The chart is not wrong; it does not have the numbers for the newer shade of yellow. In fact, there may be a second new shade of yellow. It seems they changed it again in 1999.
Here is a picture that contains both shades of yellow. The yellow and white tractor has the old shade of yellow. The deck under the red tractor is the newer shade of yellow.
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