Question

Why do clothes fade in direct sunlight?

Answer 1

Most economical clothing dyes are conjugated organic polyenes, where the absorption in the visible light is caused by pi electrons trapped in long sequences of conjugated bonds. Unfortunately, these electrons are also quite willing to absorb UV photons from sunlight, which then provides enough energy to break the chemical bonds, shortening the length of the "box" in which the pi electrons are confined. That destroys their ability to absorb visible photons, rendering them colorless.

Inorganic dyes, such as those made from transition metals -- the kind used by oil painters -- don't have this problem, and don't fade in sunlight, but there are good reasons for not using them in clothing, namely they're expensive and can be toxic.