“We know that Prokudin-Gorskii intended his photographic images to be viewed in color because he developed an ingenious photographic technique in order for these images to be captured in black and white on glass plate negatives, using red, green and blue filters. He then presented these images in color in slide lectures using a light-projection system involving the same three filters.”
Making Color Images from Prokudin-Gorskii’s Negatives
So, it seems we actually had color negatives in and around 1909. He knew the process would work at some point and the technology would catch up to his vision. Amazing.