eli5 Why do colors fade on things left in the sun?
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A coloured thing is coloured because it reflects certain wavelengths of light and absorbs the rest. All photons of light carry energy so the energy of the photons that get absorbed has to go somewhere. Mostly it gets degraded into heat but the chemical structure of some dyes are mote fragile than others. Absorbing the photon can cause it to break apart and lose its colour. Typically, colours that are based on metal compounds are stable, for instance the green or blue of copper salts or the red and ochre of iron oxides. Those depend on the electronic structure of the metal. By contrast, dyes based on organic, carbon compounds get their colour from the particular way certain atoms are joined together. If the photon breaks a bond, the structure changes and the vast majority of organic compounds aren’t highly coloured.
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