Great that you're doing your own research, dittrich. Kudos to you for that.
Here's my advice:
I wouldn't be using the PF's images for any type of color study, because:
1) the scanned images all show vastly different quality
2) there are no side-by-side examples in the same scan that would give us a good understanding of how different both paper types are.
I'm glad that they have pictures at all since it at least gives us a window into what they looked at, but they shouldn't be used to expertize our own.
PPG gives the best advice which is purchasing a cheap copy as the key reference for your search.
When you do obtain one, I'd also like to add that it's extremely important to expect potential shade differences of the same color, as these are two different spectrums that are both relevant to the search.
Meaning: if you do obtain one copy of bluish paper, do not expect every other bluish paper to be just as light or just as dark as the one you have.
This is the same as any other general issue. You will see that the papers of the same stamp can be lighter or darker, but that does not classify it as a separate type of paper.
Here is an example of two Sc#462's, one way brighter than the other:


In my opinion, this is why the term "bluish paper" is used rather than "grayish paper" since it focuses on the color difference rather than the shade.
These are just my observations. Not financial advice.