In discussions with Dr. Oscar Gaddy the subject of case colors fading came up. His explanation made perfect sense. Case colors are the result of attaching carbon molecules to the base metal through the application of heat. There are only three possible ways to dislodge these carbon molecules, friction, chemical action or sublimation and since carbon cannot under room temperatures pass from a solid state to a gaseous state, chemical action (oxidation/rust) loosens the bonds between the carbon molecules and changes the color but friction is the only possible way to destroy (remove) the case coloring.
I also had those very same conversations with Oscar. He was an engineer by training so speculation was not something he did. Oscar did his own testing using full spectrum UV light over an extended period of time and saw NO evidence that UV degraded the colors from color cased steel. As TwiceBarrel stated the colors are a mechanical property not a chemical property. I am also in doubt as to whether standard flourescent bulbs emits much if any UV light. If they do I've spent a lot of extra money to by full spectrum bulbs for a finish drying cabinet which also carry a warning not to look at them for any length of time because of danger to your eyesight.
Hey I could be wrong, and I often am, but until there is some very sound fact based data to the contrary I'll go with abrasion and corrosion rather than UV as a reason for the colors to disappear.
