Natural materials change colors often, it’s like in nature, the green grass drying takes the color of hay, it seems trivial and obvious in this case. The extracts follow the same logic, a green extract or light yellow can become straw yellow, or even brownish. We must think of the dead leaves in autumn, then in winter and with the fruits that dry …
I would rather say that a change of color is not always a sign of a change of smell, but such a change can sometimes bring a modification in the fragrance.
For an already aged perfume, whatever its aspect, no need to ask questions, you just have to feel and see if you like it or not, it’s that simple!
If you want to keep your perfume for a longer period, I suggest you leave it in the box to avoid contact to direct light, and or the sun. Also, put it in a place where the temperature is controlled and stable, could be your bedroom! Avoid humidity, like the close proximity to your shower is not a good idea.