If you've found that your epoxy hardener has discolored in the bottle, this is caused by oxidation, not UV light. The hardener will yellow in its liquid state more quickly once the bottle has been opened because it has been exposed directly to oxygen, and this will always be more noticeable in larger containers (just as how the ocean looks blue but a cup of water looks clear). It can still yellow very slightly if unopened due to the fact that the HDPE plastic bottles it comes packaged in does breathe a little, allowing oxidation 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 (i.e. it will still cure properly). It will always appear less yellow on a piece due to the fact it must be mixed with an equal part of resin thereby diluting it, and the product will be spread out in a thin layer making any observable discoloration much less observable.