Spinach adds a vibrant green color to this pesto, but eventually even this pesto will take on a darker, brownish hue. This happens when the blended basil leaves are exposed to air (or heat) for even a short amount of time. It's almost impossible to avoid oxidation, but there are a few tricks that might help. 1. Blanch the basil and spinach in boiling water for 10 seconds, then plunge into ice water. Squeeze excess water out of the leaves, then continue with the recipe. 2. Add lemon to the pesto 3. Add ascorbic or citric acid. This is what's often added to store-bought pesto. 4. Store the pesto with a thin layer of olive oil on top, then press plastic wrap against the olive oil