Gina
I don't think anyone has mentioned primitive or rough manufacture it is just the colour matching methods of 80 years ago were no where near what they are today and relied greatly on human perception.
Unless every component of the paint comes from the same quarry, factory or refinery (which they didn't) it is not sensible to believe they will be identical with exactly the same formula and will require some form of corrective intervention. Lacking computers and modern spectrographs, the human eye is all that is left.
Just sticking to Army paint.
Back in the 70's we started to get aircraft coming out of the workshop after full rebuilds with their overall olive drab quite plainly different to the other aircraft.
The workshop CO made some investigations and found the paint was coming from a different manufacturer. Still had the same nomenclature and part numbers.
This caused a wider investigation which resulted in "Colour difference is as a result of different manufacturing methods and the new colour is deemed within acceptable variation limits"
A couple of years later the vehicles being repainted at workshops were obviously different to their new colour. Not red versus blue but perceptible to even untrained eyes. Once again, correct part numbers but different manufacturer.
Lang
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