Because the sun’s ultraviolet rays, and to a lesser extent visible light, attack the chemical makeup of coloring matters, just as they damage our cells when we expose ourselves to the sun for too long without protection.
The energy of the absorbed photons can cause a chemical reaction that destroys the molecule responsible for the color. “It’s faster or less depending on the type of dye used. Dyes are less resistant than dyes, for example.” It is a phenomenon that can occur indoors as well as outdoors, but also at night, because the moon reflects part of the sunlight. To address this, manufacturers are developing more resistant pigments.
“We use additives that will act as shields. By absorbing wavelengths before the dyes, they will degrade first.” The expert continues. This allows textiles, facade plaster, and other materials that are constantly exposed to the sun’s rays to retain vivid color for longer.
According to Science & Vie # 1244