THE PHOTOCHROM GUIDE TO : American Indians. c. 1897-1899. Mexico, Arizona & New Mexico.
Invented 140 years ago in 1880s Zurich, Photochrom, a.k.a. Fotochrom, Photochrome or Aäc process, is an old school process for colourizing black-and-white negatives via the direct photographic transfer of a negative onto lithographic printing plates. The process is a photographic variant of chromolithography (color lithography).
Creatively choosing colour & then using a mechanical process to apply that colour to a black & white photo rather than doing it by hand.