Part B is actually the corrosive part of the XTR mixture. We ship our XTR in containers that have chemical application caps. If you transfer your Part B to a different container or allow debris or unknown substances to contact your Part B, it can dissolve this material and incorporate it into its mixture. This will inevitably lead to a yellowed Part B.
This normally happens when Part B is stored in a container with a paper lined lid which the fumes from Part B will slowly dissolve and contaminate the mixture. If your Part B was initially clear and has turned yellow over time, this is the most likely culprit.