Because J M W Turner used non-lightfast red pigments, the reds in this painting have faded significantly. By 1859, staff at the National Gallery of London had already noted that the vivid scarlets in this sunset had were dulled. We will never know what this famous piece originally looked like. (The Fighting Temeraire tugged to her last Berth 
to be broken up, 1838
, oil on canvas, 36 x 48, 1839.

What is lightfastness?

Lightfastness is a quality that refers to the way that paint colors age over time. Resisting fading on long exposure to light. A pigment's lightfastness/resistance to change depends on the chemical nature of the pigment, its concentration, and the medium in which it is employed.

Currently in the US, artists can know how lightfast a paint is by the American Standard Test Measure, which rates pigments in descending order from 1 to 5.

See also: How to test lightfastness