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There's really no way to tell for sure without comparing them to an expertized exemplar.
I agree. There are so many variables such as: fading (whether through sun exposure or through storage in a medium which absorbs some of the colour) and yellowing, then there are slight differences in scanning colours. The gum (as postmaster has already mentioned) also affects the colour.
Thanks for posting the stamps, Hawkstamp. I love seeing these German stamps. They have a beauty all of their own.