I remember when this question was posed to Dr. Gaddy.
His answer was that the colored layers are micro thin oxides, and vary in hardness. All are succeptible to oxygenation and wear. All coloring is fading from birth.
I don't recall him saying that UV has much effect. But, since some of the atoms of the colors can be excited away by UV wavelengths, then UV must have an effect on some colors with certain levels of exposure.
I'm sure those that have committed his works to memory can better answer this question.
As far as the best preservative, remember, no friction, no oxygen, no light. Just like the underside of a fore end.
