You do nice scans! You must be used to scanning and posting somewhere or selling. A lot of us do this also.
I grabbed the two smaller scans (file size) and cropped them so just the stamp were showing as shown below.
Lighter and darker.
This makes it easy, when you download them to your desktop, to flick back and forth between the two photos and see the differences.
You can see, even on these poorer quality scans (than the nice large ones scans) that the all of the stamp's colours, not just the shirt, are affected, one having somewhat lighter colours than the other. Look at the patch of bright green grass on the right side.
This seems to me to be a case of fading from the sun or a chemical reaction of some sort as it is not just one colour that is affected. I would love to be proved wrong.
To look at stamps it is best, I think, to start off with thinking , well there is something different about this and then to prove it correct and a valuable stamp rather than starting to think it is a valuable rare stamp to start with and then trying to prove it is not. Unrealistic assumptions.
In the second case you tend to miss (human error) or even at times ignore certain details (you see what you want to see) while in the first case you tend to notice more details that will help or not to prove it's authenticity.
One must consider all the possibilities and rule out those that do not fit, so that what is left over must be the answer. Good old Sherlock Holmes.
Even the quality of the air and chemicals (smog) found in the air could affect inks and colours. Perhaps these FDCs were printed by different printers at different locations.
Your comment about it hopefully being a constant error is not correct. A constant error is one that can be shown by any collector to have happened at a certain position on a certain plate or printing of every sheet and be numbered so. If not then the error or variety is what is sometimes known as an EFO (error, freak, oddity) that happens because of printing mishaps but not all the time on every sheet.
It is nice to have varieties as then you can point to them and say look I have proof from an issue that these things do happen. Thank you for showing.