Replacing the silver image with another metal salt.

Nickel Toning [Nickel ferrocyanide] produces a wide range of reds, bluish-greens and violets depending on the solutions used, but they are unpredictable and rather fugitive and this may be why nickel was not common.

Lead Toning [Yellow], Cobalt [Green], and Vanadium [Green] are all ferrocyanides and use the same approach but do not seem to have been used.

Gold Toning [Blue-black] used a process of "plating" the existing silver image with gold using a gold chloride solution. This is not found in motion pictures.

Selenium Toning [Red-brown], Hypo-alum Toning [Brown, but very simply done with a stable cheap solution] both seem to be restricted to paper prints.