Coffee, or more properly one of the scores of substances in coffee - caffeic acid? - is a reducing agent. In the presence of the silver chloride or silver bromide in photographic emulsion, it causes electrons to pass to the silver, allowing the metal to stand alone. It helps to have an alkali activator present as well, because this really only works at higher pH. If we use sodium carbonate, commonly known as washing soda, a pretty good alkali, with some of Starbucks' Instant Italian Roast (Extra Bold), we get images such as figure 1.
An observation is in order. At the center of this image there is an area that resembles what is called dichroic fog, a fog generated by an excess of silver halide complexing agents in the developer. We don't want to go too far into this - it's a fairly indeterminate subject anyway, the more so the farther you go into the scientific literature - but these complexing agents are often sulfur-containing compounds, where the sulfur gets exchanged for the metal, and the kinetics get kind of rapid and hazy. Thiosulfate, thiocyanate, thiourea, and mercaptans are some of the many complexing agents that work with silver halides; another way to think of them is as silver halide solvents, which is why you find them often in fixer formulations. In figure 1 we have mixed into the Starbucks a little thiocyanate. If we keep at it but this time reduce the Starbucks a bit, and change the amount of thio and sodium carbonate, we arrive at images like figures 2 and 3.
These seductive colors are produced not by adding to the mix, but by pulling back from the dichroic edge with reduced aliquots. A little practice is necessary, but is soon rewarded. In the end, after becoming the ethereal colorist you've always wanted to be, you may wonder what the point of the coffee is: metol, hydroquinone, catechol and so forth are reducing agents after all that yield similar results. Well, now you have a way to finish your cup, after it's grown cold. And in your chemigrams there just may now lurk a hint of Kaldi's goats and their supreme vision.