I have found two methods in the literature I have on making black and yellow silver colloids. One uses dextrin and the other uses lysalbinic acid, either one acting on silver hydroxide to make silver metal colloid in gelatin. All of these are done in mild alkali. I have to do some figuring to get out a meaningful formula in today's terms, so if anyone is interested, just let me know and I'll put it into the queue.

Dextrin is basically a hydrolysed starch, done by a very mild alkali. Maltodextrin is an example. Lysalbinic acid is the result of mild gentle hydrolysis of egg albumen. I can find the dextrins, but lysalbinic acid is not common today.