I'm guessing that color change is not a good thing. Not quite as drastic as Tiffany saying to whack the whole plant off and root the upper stems, what I'd suggest is along the same lines. I'd sure cut off some of the upper part of the stems and let them air dry a few days and then try to root them. If successful, that would be a good replacement in case the original plant does crater on you (which I think is a possibility). If it continues to decline, then I'd consider cutting it off low and watching to see of the stems put out new, thicker growth from the stub. I've done that with florist kalanchoes when they've aged and gotten leggy to great benefit on the appearance of the plant. I have rooted some of the cut off upper stems, but ended up giving those plants away when the stubs, after the pruning, grew well. West windows give almost as good light as the south windows in my location. Lots of light. Keep your plant really close to the window with nothing to obstruct the light and it should really help the growth.
