Fading and colour change in paints has been well-described since 1400. Shown here in examples using indigo, it wasn’t properly investigated until the late 19th century.
lightfast
When it was first used as a pigment, this vegetable dye proved reliable and lightfast. Later technique, though, resulting in it fading. Why?
In their original form as madder, derived from plants and poorly resistant to light. Refined to Alizarin Crimson, still fugitive, and a standard for fading. Finally fully ‘permanent’.
Painting is about form, light, and colour. Here is a short summary of the practical points developed in the previous articles on colour.
How pigments work, and some of their practical problems.
The Eclectic Light Company 