Why do overcooked vegetables lose their bright green color?

One of the primary reasons for the change in color when green vegetables are cooked is the change in chlorophyll. Chlorophyll has a chemical structure that is quite similar to hemoglobin, which is found within our red blood cells. A basic difference is that chlorophyll contains magnesium at its center, while hemoglobin contains iron. When plants (e.g., green vegetables) are heated and/or exposed to acid, the magnesium gets removed from the center of this ring structure and replaced by an atom of hydrogen.

In biochemical terms, the chlorophyll a gets turned into a molecule called pheophytin a, and the chlorophyll b gets turned into pheophytin b. With this one simple change, the color of the vegetable changes from bright green to olive-gray as the pheophytin a provides a green-gray color, and the pheophytin b provides an olive-green color. This color change is one of the reasons I have established the relatively short cooking times for green vegetables in the "Healthiest Way of Cooking" methods. These cooking methods are designed to preserve the unique concentrations of chlorophyll found in these vegetables.