Orange in Negative Film
Colour negative film has layers of coloured pigments that act as filters for separating the different wavelengths of light. These pigments are made of Cyan, Magenta and Yellow for the same reason that these colours are used in the printing industry.
Ideally they would let through light in even amounts but in real life the pigments absorb some of the light and, more importantly, they do so unevenly. The Cyan pigment is worst affected as it absorbs some extra green and blue, and the Magenta also absorbs some extra blue. The Yellow acts relatively well.
To equalise the effects of the uneven absorption of the Cyan, Magenta and Yellow pigments extra colour is added to each. The pigments are in effect dumbed down to the lowest colour Cyan. The Cyan and Yellow have yellow added to counter the extra blue absorbed by the Magenta. Magenta and Yellow have yellow and magenta added to counter the extra green and blue absorbed by the Cyan. The result of all this extra yellow and magenta is a orange cast to the film. The cast is uneven varying from light in the cyan areas to heavy in the yellow areas (see below). This absorption problem also exists in transparency film but extra filters are applied to bring the colours back into line within the film. But the negative film is left in its orange state as it is intended for printing in the darkroom, at which stage the orange will be filtered out.