Natural silver salts look white like salt but close examination shows they are off-white, somewhat cream to yellowish. This yellowish allows the silver salt crystal to absorb more blue light (reflects less blue). Dye additive alter the absorption - reflection of light. This is the stuff of sensitizing dyes. Dye additives plus adding impurities change the absorption and reflectivity. A pure silver salt is incentive to light. Finding a pure crystal is challenging. Adding impurities change the charge of the crystals from neutral to either + or -. This happens because impurities alter the structure of the crystal. Once a batch of Kodak film was ruined because the gelatin was contained due to the cows diet which in this case was wild mustard seed. To this day, some mustard is sprinkled into the batch. This contaminate and ups the ISO. Additions like this is called "doping". Many such containments are dye, gold, cadmium. Exposure to tiny amounts of mercury fumes up the ISO in a process called hypersensitization . Astronomers heat film in the presents of certain gasses. This hypersensitizes and mitigates reciprocity failure.