Pesto can turn a swampy brown quickly due to oxidation. To prevent this from happening, follow these tips: Chill: your olive oil, bowl and blade of the food processor/blender for at least 30 minutes Wash: your basil leaves but keep it short as this removes its natural essential oils; do not soak basil leaves. Quickly run under cold water then dry thoroughly in a salad spinner, and lay flat on a clean kitchen towel until they are fully dry. Small portions: store basil pesto in small containers and freeze to retain color. Defrost as needed.