Oxidation can change color
Small amounts of air enter a bottle during normal use and can gradually affect some materials, sometimes making the liquid look darker.
Light and heat can speed the change
Long exposure to strong light or heat can accelerate reactions in the formula, which is why moderate, dark storage is preferable.
Some materials darken naturally
Certain sweet or resinous materials can deepen in color over time without meaning the fragrance is unusable.
Color alone is not enough
If the liquid changes slightly but the smell remains normal and performance is stable, color by itself is not enough to judge the fragrance.
When should you pay closer attention?
If color change comes with a clearly strange smell, major performance change or unusual liquid condition, it is worth examining more carefully.
Related guides
Color change is an observation, not a final spoilage test. Smell, overall condition and storage history give a better picture.