I will hazard that its not UV that causes the darkening, but oxidization. Based on my experience in an art museum, UV and regular light were limited because any plant based colors faded away to nothing.
Jumping to the finish: Puts some oil on it, and let it age. The color will shift, but stay just as remarkable.
As it happens, some of the first serious carving I did was in osage orange, and I happen to have one work left. The finish, as recommended by my teacher was simply Johnson's paste wax. (His teacher used bees wax) I know I gave it several coats over a period of a few years, after which it sat in a small glassed-in end table at my parents house, never exposed to any direct sunlight. I retrieved it a few years ago. The portion that is sap wood is a deep yellow orange. Not as brilliant as the fresh wood was. The heart wood is a very dark reddish- orangish-brown. It is darker than a mahogany cabinet my wife inherited from her grandmother.
The base never had as much wax on it, and I see that some of the concave areas didn't get as much wax. They are slightly lighter, and some places still have a hint of orange. Evidently, some of the darkening is the effect of the wax aging in the wood.
I just cut a small notch in the base, and in a few less visible areas. The wood where there was little wax has little orange. Its a light tan color. There is a little more orange left under the waxed areas, but the color is still very faded. The waxed area surfaces are much nicer, having a deep rich hue. The sap wood retains some of the yellow-orange of the fresh cut heart wood. When fresh, it was rather ivory-like.
