Color fading is a real concern in many industries. I have an old munsell book (the standard for coloring dirt) from a retired geologist and it's no where near the color in new copies.

TIL there's a standard for dirt color matching.

This is not surprising. The chemical compounds used for the colours will degrade over time, either from direct illumination, or degradation by oxidation, or some other process. This will lead to subtle change in the colours, and if you care about accuracy, you'll need to replace them.