Why epoxy hardener turned yellow in the bottle?

Oxidization is when you notice that your hardener has discolored in the bottle, not UV light. 

Reasons why this happens:

*  Bottle has been opened the hardener will yellow in its liquid state more quickly since it has been exposed directly to oxygen. This will always be more noticeable in larger containers

* It can still yellow very slightly if unopened because the HDPE plastic bottles it comes packaged in does breathe a little, allowing oxidization to occur. For this reason, epoxy resins will always have a shelf life. Once a product has begun to yellow in the bottle, it can still be used and properly cure

* Once mixed with an equal part of resin thereby diluting it, It will always appear less yellow since the product will be spread out in a thin layer making any observable discoloration much less observable