Relax, Charles, over that color shift. It should not happen rapidly in NE USA. If you lived in south Florida or Texas or Arizona, it might be different I made a stereo equipment cabinet and matching coffee and hexagonal lamp tables of black walnut >30 years ago and these pieces are still in use today. The stereo cabinet was located immediately in front of an east facing window during most of this period. There is some color lightening, but that is mostly on the flat top (approx. 26" deep X 52" wide), and that is mostly due to deterioration of the lacquer finish rather than the wood itself. The original finish was a wash coat of shellac followed by a light brown stain and filler, followed by sprayed nitrocellulose lacquer which was rubbed out to a high gloss finish.
Dave R.