Turning to our old pal Harold McGee, we learn that the bright green color was really there the whole time! In raw form, the green chloroplasts in vegetables are clouded by pockets of gas filling the space between the plant's cells. When the vegetable starts to heat up, those air pockets expand and break away - leaving the way clear for us to see the pure, unfiltered green!
Unfortunately there's a small window of opportunity before the vegetables start to dull again, or even turn brown. If we're blanching the greens in water, the chlorophyll gradually starts to drain out into the cooking water. Even if we're just sautéing vegetables in a skillet, the heat can cause a chemical reaction that causes the bright chlorophyll to turn grayish-yellowy.
To keep those greens green, we can add a bit of baking soda to the cooking water (alkaline conditions slow the graying chemical reaction) or cook them in a copper pot (which also slows the reaction). We can also simply remove the vegetables at their greenest peak and plunge them into an ice bath to stop the cooking process.
What's your favorite way to cook greens?