Ultraviolet light is the color with case hardening. It will dull the colors down to a gray if left in the sun (or other UV source) too long.
There is a Sharps rifle here at the Cody Firearms Museum that is stored in a case that had a UV producing light several feet away. The resulting UV exposure damaged the case hardened surface and greatly reduced the visible color pattern on the side facing the light.
Almost all of our lights in the gallery have some sort of UV sheilding or are non-UV producing light sources.
As much as folks hate locking them up, it is often the best thing for the guns.