There is some indication that dyes were added in overcoats.
It is known that early dyed coatings were coated first and then dipped into a solution of the dye. The emulsion was sensitized by this dip/soak treatment, so why would an overcoat not work?
The German wording is unclear. They use a word which my dictionary translates as "basting sauce" or "gravy". This could mean something coated over the emulsion or something added to the emulsion.