For most oils, refrigeration is not necessary but can’t hurt. Blue oils should be refrigerated to slow the oxidation process. Chamazulene is what causes the blue color and, when it oxidizes, it will cause the oil to become green. When this happens, it doesn’t mean the oil is necessarily bad, but just a sign that some of the chamazulene has reacted with air. Extend the life of citrus oils by refrigeration. When limonene oxide forms in a citrus oil, it will cause a foul odor.