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So a couple of questions:- firstly does anyone collect or have the different shades? How distinguishable are they? Are they so similar that you can hardly tell or are they quite distinct?
In most cases, the Scott listed color varieties quite distinct and are fairly easy to distinguish, especially when compared to some shade varieties of earlier issues.
The problem, as you alluded to, is that online scans can be so "off-color", that unless there's a single side-by-side scan, you really cannot be sure of what you are getting.
I'm not good with IDing colors of single stamps. However, when you have a bunch of them together, the Scott listed color varieties are pretty easy to spot (especially among mint stamps). If you have to scratch your head and wonder if I have a real color variety or just a slight production variation in inking composition, then you DON'T have the Scott listed color variety.
Also, for most of the Prexies, these are actual inking changes, not just anomalous inking problems. That is why most of the color varieties have the same catalog value.
It would probably be worth your while to examine any online bulk lots of Prexies that are sorted by Scott number, as a practice.
Have fun! The Prexies are a great series!

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