As in paint, dye, ink, etc
The UV in the sunlight degrades the dye, the same way it can damage your skin and give you sunburn. The energy from the photons can actually break bits off molecules – and since dye molecules tend to be a bit ungainly and fragile, this is relatively common. The broken molecules don’t absorb/reflect the same wavelengths of light, so they no longer work as dye.
Ooh, UV light breaking down pigments, is that kinda like how laser tattoo removal works?
Sunlight contains high-energy light. We call this light *ultraviolet*, because it’s past the violent end of the spectrum of things we can see.
Ultraviolet light has as much energy as a typical chemical bond. What that means is that absorbing UV light can *break* a chemical bond, splitting a molecule into pieces and potentially forming different compounds. Colored objects usually rely on chemical dyes that are made of molecules of a particular color, and if you destroy those molecules, the color fades (or changes, if they’re reformed into new molecules that are colorful).