Semi-long time lurker, first time poster here. I'll take a stab at your question. I would guess that the lignin (a compound found in wood and thus paper) in the chip insert is oxidizing due to sunlight. This is what causes newspapers to turn brown over time. Various chips could have various amounts of lignin (more lignin=more browning) based upon the makeup of the paper used. Check the link below for more, especially the "Light" section. This is merely a conjecture, I have no real expertise in the subject.