The chemicals used undergo changes over time which alter the colours. This process is accelerated by light, pollution and humidity. In most cases the material that makes the actual image is silver metal. The amount, the size and shape, and the way the silver particles are mixed in the emulsion, determines generally speaking, the tone and durability of an image. A second factor is the composition of the developing solution, which influences the appearance of the silver particle.
Silver tends to react with sulphur, which is found in fixing solutions.
When prints are not rinsed out well enough after fixing, the image will fade. This reaction is irreversible and can go very rapid if large amounts of sulphur are present. The only remedy is to copy the image and store the original in a cool dry place. Most colour photographs, negatives, prints or transparencies are hard to keep. Because the colour-components are very unstable they tend to fade rather rapidly.