He accomplished this with a clever camera of his own design, which took three black and white photos of a scene in rapid sequence, each though a differently colored filter. His photographic plates were long and slender, capturing all three images onto the same plate, resulting in three monochrome images which each had certain color information filtered out.
Tsar Nicholas II fully supported Sergei's ambitious plan to document the Russian Empire, and provided a specially equipped railroad car which enclosed a darkroom for Sergei to develop his glass plates. He took hundreds of these color photos all over Russia from 1909 through 1915.
There was no means to develop color prints at that time, but modern technology has allowed these images to be recombined in their full original colors. The U.S. Library of Congress purchased all of Sergei's original glass negatives from his heirs in 1948, and in 2001 a beautiful exhibition was produced to showcase Sergei's photos, called The Empire that was Russia.
Once the black-and-white film base was developed, the dyed starch layer which had acted as many tiny color filters when the photo was taken now did the same task in reverse, giving the color back to the underlying image. The technology was a bit crude and grainy, but it was able to capture full color images which turned out looking rather impressionistic.
Because of the efforts of the French army photographers, there are beautiful color images of soldiers in the trenches, military equipment, ruined buildings, and villages, among other things. Autochrome plates age remarkably well due to their construction, so many of the originals are still in pristine condition today.
Autochrome remained as the primary color photograph medium until Kodachrome was introduced in 1935, and Agfacolor in the following year. Aside from Kodachrome, most modern color films are still based on the Agfacolor technology.