Color Differences In 3c Wash #64A Or 64B?

I am never going to fully understand what a pink stamp actually looks like. I understand there are many color variations in 64. But I am curious if anyone thinks the fourth stamp in this row is either a 64a or 64b? Or is it just faded? I know this question gets asked often but I can't tell.

Also, an explanation of what professional graders use to ascertain the color of these stamps or any references for the color of these stamps would be appreciated.

As always, thanks in advance for any insights.

I was always told the 64-pink looks like the 9 cent presidential.

Let me suggest a different approach that ...


and first start with the assumption that you have a Scott 65 rather than a 64, particularl if you have a cover that falls into 1862 or later. There are published articles on the color chronology of 64/65.