In general cook greens just until they turn soft and tender while still retaining a vibrant color. And this is usually between 5 to 10 minutes of cooking time. Adding a pinch of baking soda to the cooking water also makes it alkaline and helps retain the green color, but this is usually not necessary. For the soup here, we just watched the cooking time, giving us a bright green soup that retained its’ lovely color even after reheating a few times.