Well.....it is black and white, or at least monotone, but of course Gum Bichromate comes out in whatever colour dye or pigment is used.
A Hurdy Gurdy Player at Rochester Folk Festival
A slightly modernised Victorian Gum Bichromate process, from a normal digital shot, reduced to a Black and white negative in photoshop, then printed as a negative on ordinary lightweight inkjet paper.
This is then contact printed in sunlight onto a sheet of art grade paper, heavy grade cloth content, which has a sponged on coating of a saturated solution of Gum Arabic (water soluble glue), mixed with saturated Potassium Dichromate (Potassium Bi-chromate is not used now), plus red and brown water soluble art pigment, I use Windsor and Newton pigments.
It is dried off in the dark, but can be quickly handled in light, to contact print under glass, the inkjet image on to the Gummed paper. With the paper negative it takes about 20 minutes in bright sun to print out the image, which is then washed out uuder running clean water to reveal the final waterproof hardened gum image. Bright areas are washed with a brush to lighten them more.
If not deep enough in tone it can be re-coated and re-aligned for a second exposure, even changing the pigment.
It results in a virtually fade free and archival quality image, they do not fade within reason.
It is a print process anybody can do as long as they have a computer printer, it does not need a darkroom.
Obviously, I have re-photographed the final Gum Bichromate print back to a Digital image for the forum.
The reproduction is deeper in tone than the real thing, not much, but if printed lighter in photoshop as a Jpeg, it losses the tone gradient in the original. This particular gum bichromate was double coated, and exposed in register twice under UV lamps.
Most single exposures are weak and may only suit very ethereal landscapes.
Full colour prints can be done, using colour filtered paper negatives, and the three primary colour pigments one after another. It is the equivalent of the 3 strip 1930's Technicolor cinema film process from Hollywood, but without the bright colours. They used a dye transfer from a Gelatine Matrix
Results are a bit unpredictable in strict colour terms, but are usable and correct in deeper tones. wrong colour balance can be corrected by further extra layers or manually applied colour washes.
The main advantage of all gum prints is the archive quality, nothing deteriorates faster than equivalent art pictures, and they could last centuries. The paper used must be good acid free art heavy grade paper, or rag paper, and should be a rough finish, not fine smooth, as it aids good coating of the wet gum.