Because J M W Turner used non-lightfast red pigments, the reds in this painting have faded significantly. We will never know what this famous piece originally looked like. (The Fighting Temeraire tugged to her last Berth
What is lightfastness?
Lightfastness is a quality that refers to the way that paint colors age over time. A pigments 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