Oxidizing, the interaction of certain cut produce or herbs with the air, can cause browning of the surface. Think of guacamole left out or even in the refrigerator if not sealed in an airtight container. The same can happen with pesto recipes. Different types of basil can cause the basil version to brown almost immediately. It still tastes great but is not a pretty vibrant green color. Or when left in the refrigerator this can happen. It's is still good to eat (provided it has not been left in the refrigerator over 4-5 days).