Photochrom is a process of hand-colouring photographs from a single black-and-white negative with subsequent photographic transfer onto lithographic printing plates. The process is a photographic variant of chromolithography (color lithography). Because no color information was preserved in the photographic process, the photographer would make detailed notes on the colors within the scene and use the notes to hand paint the negative before transferring the image through colored gels onto the printing plates.

In 1898, the US Congress passed the Private Mailing Card Act which let private publishers produce postcards. These could be mailed for one cent each, while the letter rate was two cents. Publishers created thousands of photochrom prints, usually of cities or landscapes, and sold them as postcards. In this format, photochrom reproductions became popular. The Detroit Photographic Company reportedly produced as many as seven million photochrom prints in some years, and ten to thirty thousand different views were offered.

From Library of Congress.