I see lots of views of your post, but no answers, so here's my opinion.....
You don't. Especially on a used stamp. Too many things can have happened over that length of time to change the color. I guess you'd have to send it in for expertizing somewhere.
Or if you were fortunate enough to be somewhere where there was a large stamp club, someone might have a reference set with many of the different shades so you could do a side by side comparison.
I agree with Dave regarding the need for expertizing.
The most recently archived 1847USA page on web.archive.org for the 1861 issues indicates that about 100,000 of the Scott 64 were issued as compared to about 1.8 billion of the Scott 65.
Assuming those numbers are accurate, that suggests the odds are overwhelmingly stacked in favor of the stamp being a #65.
Amos Publishing (the Scott's catalogue people) publish a color guide specifically for 64 and 65 (three other issues also--10 and 11, 267, and 279B, and one other I think). It has Munsell color chips for the relevant colors.
I have a copy of the guide. That being said, I have a difficult time assigning colors. Just when you say to yourself "that's it" doubt then comes in and you aren't really sure. Also on a stamp that old, the color will almost surely not be exactly the same as when issued.
The advice already given---get it expertized certainly is good advice.
I believe you can only really compare mint examples. If it used it has almost certainly been in water and this is bound to affect the colour in some way.
As for having a stamp expertised, your own knowledge will create expertise and in the end it is only one person's opinion.
The value of these certificates has been reduced markedly in recent years due to (at the very least) some careless assesment of the stamps offered for consideration.
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