"Pesto will lose its vivid green color and turn brownish when exposed to air but there are a few tricks to prevent that from happening when storing pesto in the fridge. First, we try to store it in a taller, narrow container or jar to minimize the pesto’s exposure to air. Pouring a thin layer of olive oil over the top of the pesto before closing the lid will also work, but we find that eventually makes the pesto overly-oily as it is mixed in, if you are using small amounts out of the jar. As an alternative, we have found that placing a piece of plastic food wrap directly against the top of the pesto before placing the lid on the jar also works well and avoids the ‘overly-oily’ problem...."