It's best to eat your pesto right after you make it since avocados have a tendency to brown from contact with air (oxidation). If you need to make it ahead or store leftovers, make sure to tightly pack it into an airtight container with little space to slow the oxidation process. You could also pour a little neutral oil (like avocado oil) on top of the pesto or a layer of water (pour it off before serving) to create a seal.