Those pictures were likely hand colored. My grandfather was in WWI and a photographer. He taught me how to hand color black and white photos.
I agree. Hand colored. Not as skillfully done as the Civil War ones, which are REALLY good.
LOL! Not likely.
Hildebrand used the autochrome process, patented in 1903 by the Lumière brothers, who are also credited with making the earliest videos.
You can google for more of his work. He was the real deal.
Or have a look at these.
Or check out the even earlier photos of Prokudin-Gorskii, a Russian photographer, who used a process of his own invention, but which worked well enough that the Czar outfitted him with a special rail car and sent him out to photograph All Russias.