Don't get caught in the often quoted story that sunlight causes fading of case colors. I have ran some short tests on the exposure of case color Smith's to sunlight. Quoting from my article in Volume 18, Issue 1 Winter 2020 of our Journal in which the guns were partially wrapped to prevent damage:

"In summary, the two day test exposed the guns to 12 hours of sunlight with towel temperatures ranging between 82 and 110 degrees. At the time of retrieval on both days, the guns were very hot to the touch, estimated to be well over the capacity of the thermometer that tops out at 120 degrees. In addition, the wraped guns were exposed to sunlight for about a month in front of a window, before the outdoor exposure. However, the window glass would have filtered out much of the ultraviolet light waves...

...It seems anti-climatic, but a close examination of the guns before and after sunlight exposure shows no difference, both in the photos and in hand...

...If sunlight doesn't lighten the colors, then what does? THe answer is normal wear as suggested by Oscar Gaddy, who, in a statement attributed to him said something like, If you want to lighten the color case-hardening, soak a pair of gloves in rottenstone and go hunting..."

The reference to wrapping is primarily concerned with covering the wood to prevent damage and to more easily detect change in color of the case hardening. THe window glass is regular material as far as I know, nothing special.

When I wrote this article I researched the literature and found a lot of rumors about sunlight problems but never any definitive proof. So I don't concern myself with exposing any case-hardened gun to sunlight but I am mindful of careful use and after use maintenace!