1986 Liberty Stamp, Scott 2224, Color Differences

Approximately 2 weeks ago in this thread: http://goscf.com/t/69524
I noted that the Liberty joint issue with France showed two different color blues even though the inks were identical. These are the two stamps from my collection:



I had stated that the difference in color was due to press pressure. Not quite right. The difference is due to how the plate was wiped. Here is the explanation by Fred Boughner in his book "Linn's U.S. Stamp Yearbook 1986":




Color is not easy.

Despite seeing an explanation in-print, I am suspect that wiping differences (a physical process) can create such a color difference (a chemical composition difference) in the finished stamps. It just doesn't add up.

I think it is because of more thorough wiping done by the US D press. It is harder to see in person, but the scan above shows a bluish tinge to the background of the French stamps. I brought up/the blue in the US stamp a little and found there is even a bluish tinge to the background there. The result overlaid over the two blocks:

I think the blue matches the French issue well, as does the red. The slightly more bluish background might be responsible for a different perception of the inked colors.