Also, overworking the basil in the processor heats the mixture, breaking down the chlorophyll, which is the source of the green. Some chopping is necessary for the herb's essential oils to release their flavor. But a prolonged puree turns things muddy.
The solution: Pulse; don't puree. Place whole basil leaves, garlic, cheese, nuts, and about 2 tablespoons oil in the bowl of a food processor, and pulse 2 to 3 times or until a rough paste forms.
Repeat with more oil, pulsing after each addition, just until smooth. You can also add a couple of tablespoons of parsley or spinach to help preserve that bright green color. When it's time to serve, top hot foods with pesto in the serving bowl at the last minute, rather than in the skillet. To store, place in an airtight container, and place plastic wrap directly on the surface of the pesto. You can also drizzle on a top layer of oil to help prevent oxidation.