I did some experimentation a while back with a product made for leather auto upholstery called Meguiar's Gold Class Rich Leather Cleaner/Conditioner. It contains a sunblock, so I dyed a couple of pieces of veg-tan with Angelus turquoise (any blue-ish colors are notorious for fading quickly in sunlight), treated one of them with the Mequiar's, left the other untreated (for comparison), and set them both in a sunny window for a couple of months. They both got exactly the same exposure to direct sunlight, which was several hours a day.
This photo shows the results. The patch treated with Meguiar's is the one on the left. Although some fading occurred in both patches, the one with the Meguiar's had much less fading.
I also ran similar tests with different finishes - wax, lacquer, and acrylic, comparing pieces of dyed leather with a finish and without it to see how much fading occurs compared to an untreated patch. None of these finishes has any added UV protection, but they do differ in the amount of fading that occurs to the color from exposure to UV radiation.
I was very surprised to find the acrylic (i.e., supershene, satin shene, resolene, etc.) was the finish type that offers the most protection against fading due exposure to UV radiation. The wax finish seems to have made it worse! The lacquer finish had no apparent effect that I could tell.
I've also done similar experiments with water-based dyes, and though they, too, will fade in sunlight (especially the more bluish colors), they hold up better than the spirit dyes do in general.
Kate