Why can faded pigments in an old painting usually not be restored to their original colour?

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Inside the ongoing preservation of the Vatican — Art conservation and restoration Learn about the restoration of tiles in St. Peter's Basilica, Vatican City.

In the field of art conservation and restoration, it is generally impossible to restore faded pigments to their original color because the chemical alteration of the material is permanent.1

Many historical pigments—such as certain vegetable-based "lakes" or copper resinate—undergo irreversible chemical changes when exposed to light over long periods. For example, copper resinate, which was commonly used as a transparent green, chemically transforms into a deep chocolate brown after prolonged light exposure.1

Because these pigments have been fundamentally altered at a molecular level, they cannot be "reversed" to their initial state. Consequently, the primary goal of conservation is to arrest further decay by limiting exposure to light, particularly ultraviolet radiation, rather than attempting to chemically revert the faded colors.1