Some pigments are stable enough to withstand the ravages of light. The iron oxides and the heavy metals such as cobalt and cadmium are composed of solid particles of material unaffected by UV light.
That’s why barns are a brownish red: Iron oxide is one of the cheapest and most lightfast pigments. Farmers used to make the binder out of milk-based proteins.
Carb-Othello pastel pencils and Caran d'Ache colored pencils did very well when subjected to the same eight months of sunlight that faded the markers earlier. 
Even though watercolors can sometimes be more fugitive than oils or acrylics, these pan colors from a portable set by Schmincke hardly changed at all.
And the oils did OK, except for certain pigments. We'll look at that question tomorrow.
