Over 50 recognized shades of #64, 64b, 64a, 65, 88 and 94. You can't use color identifiers-- well I guess you could, but you wouldn't have provenance there.
Only way is to match the shade with the year and time of year with covers or with year-dated cancels on the stamps. Many different articles in "The Chronicle" which you can find on the USPCS website on the shades, and you can also find a listing and by year date.
We've gone over this a lot in the last 5 years or so. The shades, by year and time of year were described by Mike McClung, and I've posted a lot of the shades several years ago-- and can only do that because I bought a lot of covers from Mike, and he was nice enough to ID them for me.
If you look in the archives for some of my posts, you'll find the shades, and references to the articles that I'm talking about here.
Good luck in your searches! Again, best to do this on cover, with year dates. Also, you can find many for sale on eBay by Jack Daley, who is one of the "shade collectors" out there, and has ID'd many by looking at the pixels with Photoshop.