Here's why and how to tell:
* Expiration Date: Most hair dye products have an expiration date printed on the box. This is the date after which the manufacturer doesn't guarantee the quality or effectiveness of the dye.
* Changes in Texture: If the dye has thickened or become clumpy, it's likely past its prime.
* Changes in Color: The dye may change color, becoming faded or darker.
* Strong Odor: A strong, unpleasant smell could indicate the dye has gone bad.
How long does it last?
* Unopened: Unopened hair dye can last for several months to a year, depending on the specific brand and formulation.
* Opened: Once opened, hair dye should be used within a few months.
Important Note: Always refer to the instructions on the product packaging for specific storage and usage guidelines.
For best results, it's always better to use fresh hair dye. If you're unsure about the quality of your dye, it's best to err on the side of caution and purchase a new bottle.