I got the wild idea while in college to color in black and white negatives with felt tip markers for printing on color paper. I used opposite colors, cyan for red, blue for yellow etc., on the emulsion side of the negative and would print on color paper. It was a total pain in the butt trying to get the grays to print neutral without the orange cast of a traditional color negative but I persevered and got them to work.
Flash forward more years than I care to admit and I discovered these negatives while I was digitizing and archiving some of the early work that was important enough to keep in this realm. Imagine my surprise and outright glee when I found them and remembered the process. Of course now with our new technology scanning them in "48 bit color" instead of trying to print on paper that needed the orange cast of a kodacolor negative its all done automatically. Love it.
I just may have to break out the darkroom equipment and start shooting in black and white again to continue this experiment. Of course I will be scanning the negatives, not printing them as I don't have a place to set up a darkroom.