The carbro process is based on the fact that when a dichromated colloid (a dichromate salt suspended in pigmented gelatin) is placed in direct contact with finely divided silver, the gelatin hardens in proportion to the amount of silver present. It is closely related to the carbon process. The main difference is that the carbro process the gelatin was hardened by squeegeeing the sensitized carbon tissue against a silver bromide print. In the carbon process the gelatin was hardened by the action of light. The name “carbro” is derived from the combination of carbon and bromide. 

Three separation negatives were made, each through a red, green or blue filter, which were then in turn used to print three silver bromide prints. Three carbon tissues, consisting of cyan, magenta, or yellow pigmented gelatin on a paper support, were sensitized with a potassium dichromate solution. Each tissue was then exposed by squeegeeing it in contact with a silver bromide print made from the corresponding additive color separation negative. For example, the yellow pigmented tissue was squeegeed against the print made from the blue separation negative. The gelatin hardened in proportion to the amount of silver present in the print. The shadows and mid-tones hardened and the highlights remained water soluble. The hardening action happened from the top down so that in the mid-tones the soluble gelatin was under the hardened portion of the gelatin. The exposed tissue had to be transferred to another, temporary, support before development so that all the soluble gelatin could be removed preserving the mid-tones. After each exposed tissue was transferred to a temporary support, each was developed in a warm water bath which dissolved the soluble gelatin in the highlights resulting in a pigment image in relief.  Each relief was then transferred in succession and exact registration onto a second temporary support beginning with the cyan relief, then magenta then yellow resulting in a full color image. The combined image is then transferred a third and final time onto a final support consisting of paper coated in a thin layer of hardened gelatin.  The final layer order is yellow at the bottom, magenta, and cyan on top.

Three color carbon and color carbro prints are nearly impossible to distinguish from one another. Carbon printing began to decline with the introduction of carbro so prints after 1930 are likely carbro.  One major advantage of carbro was the ability to make enlargements because the tissues were exposed in contact with a silver bromide enlargement rather than contact printed. Also, it was easier to make adjustments in contrast which affected color balance to the silver bromide prints then it was to separation negatives. The carbro process was commonly used for commercial purposes such as advertising between the 1930s and 1950s until it was displaced with the introduction of Kodak Dye Transfer. Carbro as well as carbon were both used to make monochromatic art reproductions as well.