Photodegradation
Photodegradation is how light from the sun breaks down colors and things.
Think of it this way
A piece of colored paper left on a sunny windowsill fades over weeks. The sunlight breaks apart the tiny bits that give the paper its color, little by little. A matching piece kept in a dark drawer stays bright. The sun's rays act like tiny hammers, slowly knocking the dye apart until the color is gone.
Explaining photodegradation by grade level
Put colored paper in the sun for a few days. The paper starts to fade and lose its color. The sun's light is strong enough to change things. Shade keeps colors from fading.