Fade of Dyes

1 - Low formaldehyde concentrations in some colour process stabiliser solution can shorten the life of some incorporated coupler colour dyes.
2 - The irradiation from the lamp the projector lamp. Too high a temperature on the frame can burn the emulsion and deform the base as well. The high ultra-violet content of some projection arcs also fades dyes, and this can be seen as a faded central spot or area in each frame. This is the most common cause of fading of coloured films from the silent era.

3 - Storage conditions, particularly temperature and humidity.

4 - The chemical and physical properties of the emulsion matrix material. Surprisingly little is known about the differences between the different gelatine materials in this context, which are chemically a very mixed lot.

Toning is the process of changing the silver image for another image chemical. Some alternative images are much less stable than metallic silver, and most fade to some extent. Iron-tone blue, also called Prussian blue, fades slowly with the effect of ultraviolet light, from projectors, for example, and Prussian blue also appears to darken considerably with age. Some of the orange tones, especially the common Uranium ferrocyanide (ferricyanide) appear to darken and to change colour, to a duller, redder hue. These changes result in considerable changes in the rendering of all the 1930's 2-colour systems, such as Cinecolor. Some toned images have faded unevenly, surely the ultimate problem of restoration, one for which there is no simple conventional photographic solution.
Sepia toning, the conversion of the silver image to silver sulphide, which has a warmer, browner hue than neutral grey of silver on very thin prints and colder tones on dark prints probably also changes, but is known to be impervious to residual fixer salts. Eventually the colour becomes more neutral and the colour effect is lost but the density is usually maintained. On the other hand most toning is quite difficult to identify after all these years and not enough is known about the original colours to predict the changes that have occurred.
Tinting, colouring of the base or the emulsion overall, is generally more stable than most toning, with some notable exceptions. Heat from projector lamps has the effect sometimes of reducing colour saturation especially of some reds. In general it seems that yellows and reds were more stable than greens and blues. The red tint dye Amaranth is immediately recognisable [by densitometry] even after 80 years. Some of the old blue dyes fade unevenly in a most characteristic and unpleasant way. Tinting of the base seems to be more stable than the laboratory processed emulsion tinting.
Stencil colouring and brushed on colours are generally as stable as the tint dyes - indeed Pathe in France used the same dyes for both. Technicolor dyes, for a long period Metanil Yellow, Rhodamine and Patent Blue, which is, after all, a little like stencilling, are also quite good, but there were periods when poor stability seems to have been a problem.