In an article she wrote for Poultry Success Magazine in 1917, Nettie stated "The Buckeye should be as much darker in color than the accepted Rhode Island Red as the Rhode Island Red is darker than the Buff breeds. Their plumage should be so dark as to male as to look almost black in some lights, garnet red being as near a description as I can give."
And until now, I had never had any images from that era to go by, to determine the actual color of the RIR's she was comparing them to. But I found one! This plate is from a book from 1912*, by F.E. Wright, and shows a number of different breeds of the time, including the RIRs at the middle right.
(Note this image has not been color corrected in any way, what you see is what was scanned. Just a small amount of retouching was done to remove extraneous dirt.)
It's fascinating to compare the color of those birds with that of the other Buff birds on the page, as they really aren't that much darker!
But then when we look at the BB Red Game bird (yes, in that picture it's a bantam, but shows the accepted color for what were then called "Indian Game" birds, which she says she used to create the Buckeye.)
That color is a darker, glossier red than the Rhode Island Red of the day. And that may be where Nettie got the color she was looking for at the time for her Buckeyes, that "garnet red" color she loved.

Yes, in some light, properly colored Buckeyes look almost black. The photo at the left (not the best focus, because I took it with my phone) shows a cockerel of mine at a show this past fall. He looks "almost black" in that light. But in daylight he is a lovely, glowing red, just like the garnet Nettie was striving for.
It's also important to note that the Standard of Perfection of today calls for a color that is "rich mahogany bay." Certainly, the old RIR's were nowhere near that color. They were much lighter than the "garnet red" Nettie bred for. And just because RIR's have gotten darker over the years, doesn't mean the Buckeye should go any darker than what the Standard calls for now, despite what some folks say is "Nettie's vision." We can see illustrated above what she was striving for, although bear in mind there are different colors of garnets out there too, all you have to do is Google "garnet red" and click the Images tab to see the different shades available that all fall under "red."
But I think it's important for folks not to get lost in what they think Nettie wanted, after all, no one has a Ouiji Board they can use to talk to her and get a photo of an exact bird she'd like. But we can use illustrations from the time she was creating and breeding the birds to guide us, so we don't lose our way.
*Note, the original of this image is no longer under copyright, but my retouched version now is under my copyright. Please do not right-click or otherwise "share" this image. Thanks.