Interesting, because I can see the difference right away, with the Pocketbook ERA and the black ink seems to be a slightly faded color without the black depth as we can see, for example, with Tolino or Kobo.
I am quite surprised that Pocketbook allegedly wanted to optimize the screens of the ERA model, and yet another Inkpad 4 model suffers from the same problem.
PB ERA model after 2022:
You can see it here ( Onyx Boox Leaf 2 ). It is the same E-INK screen, but the contrast is different due to the additional layer in the black model.
Look for time "3.10" :
https://youtu.be/sB12gbEI6J4?feature=shared&t=174