The original stippled image was re-photographed on an 8 by 10 sheet of high contrast film called Kodalith (used in the printing industry for making printing plates). This resulted in a negative, shown below. The image is actually the clear areas on the film
Remember that the entire sheet of film has a gelatin coating. The film negative was reprocessed to remove all the gelatin coating in the black areas of the film. The gelatin holding the remaining clear image was not touched. Food dye was then applied to the film. It was absorbed by the gelatin and was easily wiped off the film where the gelatin was removed. The result was a coloured stippled image on film.