Most colour films (and particularly those in present need of restoration) are constructed as in fig 1.

The layers of film are all sensitive to blue light. The outermost layer is sensitive ONLY to blue light and no other colour. The other layers are sensitive to blue light as well as green (for the green layer) or red (for the red layer).

Light passing through the blue sensitive layer must have the blue component removed so that it does not expose the inner layers. Since white minus blue gives yellow light, a yellow filter is used to exclude blue from the inner layers. During development, the yellow film is washed out of the emulsion.

When the above film is developed either via a negative or by a reversal process, the following can be assumed for an ideal film:

The intensity of white light passing through the film will be modulated by the intensity of the pigments in the three layers in such a way that the original intensities of the three primary colours red, green and blue are restored to those of the original recorded image. This means that if blue light was intense, then small density of pigment in what was the blue sensitive layer is present in the film. The same applies to the other colours. Please note that the dyes in the film must be subtractive colours (Cyan, Magenta, Yellow) since the layers of dye subtract from the white light passed through the film during projection. The conversion from RGB to CMY is as below, where 100=100%:

Cyan = 100 - Red (Red = 100 - Cyan)
Magenta = 100 - Green (Green = 100 - Magenta)
Yellow = 100 - Blue (Blue = 100 - Yellow)

Colour degradation is caused by the fading of the pigment layers due mainly to exposure to the air. Since the blue-sensitive layer (yellow dye) is in direct contact with the air, it is expected that this layer would fade first. Fading of the next (green -sensitive) layer (Magenta dye) would be less, but still greater than the inner layer (redsensitive - Cyan dye).

In the following diagram, I have assumed that there has been a 50% fading in the outer layer and a 25% fading in the middle layer, with no fading in the inner layer. This model is not absolutely true, but shows the sort of degradation that might be expected in a colour film. I have also indicated the colour degradation if the amount of fading of the yellow is twice that of the cyan and the magenta has faded half-way between the cyan and the yellow. In practice, the degree of fading is not linear within a layer and the relationship of fading between layers is somewhat unpredictable, but the example demonstrates the principle.

Fig 3. shows an undegraded picture. Fig 4. shows the same picture subjected to panchromatic fading (all colours fade at the same rate). Fig 5. shows the same picture with only yellow and magenta faded according to the first model. Since all colours would be expected to fade to some extent, Fig 6. shows the second fading model above.

In practice, these are the sorts of colour degradation that we have experienced in sample footage that we have on tape. (source: ABC video archives). As a first approximation, this could already be used to restore film that is not too badly degraded.