The tricolor process grew out of advancements in color theory made during the 19th century. The basic premiss was that the mind interpreted all colors from only three primary hues, and thus a full color image could be printed from only three plates. The first printed tricolor image was made in 1877 but for many years the process remained much better in theory than in practice, largely due to the poor color sensitivity of photo emulsions. A workable panchromatic film emulsion was invented in 1881, and even though it still only produced a black & white photograph, different color filters could be placed over a camera’s lens to accurately separate out different colors of the spectrum within the same scene. When transferred onto plates and printed in corresponding additive colors, it created the illusion of a natural color photograph. It was an expensive process when done right, but it could still produce full color images, even if unnatural from only three plates all printed off the same press from any image source filtered or not. This made it attractive to many printers in the first half of the 20th century who utilized this method in less time consuming ways that produced very unnatural looking pictures. A pallet of RYB primary colors would usually be utilized but in most cases it would be the retoucher rather than a filtered photograph that determined the placement of hues. They would all be rendered in halftones positioned at precise 30 degree angles to each other. This placement avoided unsightly interference patterns while creating a rosette pattern that was acceptable to the eye. These interim measures would continue to be used with lithography and line blocks until replaced by process printing in the 1940’s.
Use the link below for more insights into this process.