Once avocados are cut open, exposure to air will quickly cause the flesh to turn from bright green to brown. Acidic lemon juice is added to the deviled egg filling to slow the oxidation of the fruit. The ascorbic acid (vitamin C) in citrus is an antioxidant, reacting with the oxygen in the environment to slow down the rate of enzymatic browning. This is only a temporary fix, so it's best to enjoy this dish within 24 hours.