Once the hardener has been manufactured it starts a process called oxidation which will cause darkening of the hardener over time,
due to exposure to oxygen. Yellowing can still occur even if unopened because the HDPE plastic bottles it comes packaged in allows the product to breathe and expand.
But no worries, even if the hardener has a slight yellow tint it can still be used, and it will cure properly. Once it is equally mixed with resin the final product will cure crystal clear.
**If your epoxy hardener is a dark yellow or orange color, you may have passed the expiration date, the product has been stored in extreme temperatures or exposed to UV rays. Your epoxy should still cure properly, but it's important to test the color of the hardener mixed with the epoxy as it can cause a discolored finish if the hardener is a dark yellow or orange color. Most customers use the expired product for use with dark pigments and purchase new product for lighter pigments or a clear finish.