I’ve always known that colours can fade in direct sunlight, and when we produce materials for outdoors or window displays they are typically given protection, but I’ve never really given that much thought about why colours fade in the sun. I noticed someone posted the question on Reddit, and the answer is pretty fascinating, and way more complex than I realised:
The short answer is that sunlight causes irreversible changes in the compounds and molecules that gave a material its color in the first place, in a process we call photodegradation. As a result of these changes, the material gradually loses the ability to specifically absorb (and reflect) different parts of the visible spectrum, creating a faded appearance.
To look at things a bit more closely, the color of the dyes used in clothing, colored paper, etc., depends on the detailed arrangement of atoms (and their bonds). For example, take a look at the structure of this common dye called Orange II. Notice how there are two rings with alternating double bonds (we call these aromatic rings), linked by a nitrogen-nitrogen double bond. This extended ring structure is what allows this compound to strongly absorb blue and green light and to reflect the yellow-red part of the spectrum. Now as you might expect this compound will initially have a vivid orange color. However, if you expose this dye it to light, over time this color will go away and you will be left with a product that is mostly clear, as shown here.
So what exactly happened that made the color go away? Well, in one word photochemistry happened, namely light promoted chemical reactions. Now these reactions can be very messy, for example here are some of the pathways you may see, and in fact multiple reactions often play out at once. However, what is important is that these reactions destroy the initial chemical structure of the dye and break it up into smaller bits. As a result, a layer of this material will slowly lose its ability to strongly absorb visible light.
Now this was just one specific example, of course in other cases dyes will have completely different structures and will be destroyed by different mechanisms. However, quite often the net effect of these changes is that they will reduce the ability of molecules and compounds to absorb in the visible part of the spectrum. As a result, as you allow colored materials to be exposed to light (and air), often you will find that over time they will lose their vivid color and will take on a dull off-white appearance. This effect is very similar to old-fashioned chemical bleaching.
The whole discussion is pretty fascinating.