Many things, when left in direct sunlight for an extended period will change color. Hair, leather, and bones all fade while some woods like cherry will darken. What is it about the sunlight that causes these items to change color? I would guess that the energy from the photons causes some sort of reaction but I can’t imagine what that reaction would be, other than exciting the electrons a little. Do the molecules actually break apart or change their internal structure? Thanks.
UV rays destroy cells, molecules, etc. The rest is details…specifically, how many different types of cells relfect light to product a certain color, and how those are destroyed by UV light, so that less are there to reflect the specific light color that produces the color of said object.
Brief answer… many coloured organic compounds consist of fairly large molecules containing double bonds. Such molecules are called chromophores. The electrons in these double bonds can resonate when struck by photons, thus absorbing energy. They absorb strongly at a specific wavelength - if they absorb at the blue end of the spectrum then they will reflect more red light and so will appear red.
Now, UV light is quite energetic. Double bonds are unsaturated, so they have the potential to break open and combine with other atoms and molecules. The UV light can provide the kick of energy for these bonds to break and recombine. The end result is that the chromophore molecule breaks down into other compounds which do not have that electron resonance, and so do not absorb light in the visible spectrum. Hence, no more colour.