Have you ever wondered why steamed vegetables turn an even brighter green initially, but then will turn this dull green if you cook them too long?
In raw veggies, gas trapped in tiny spaces between cells clouds our view of the cell's bright green chlorophyll. Heat from steam or boiling breaks down the cell membranes, causing the gas to escape. Liquid from the cell flows out into these spaces, allowing a clearer view of the bright chlorophyll, causing your veggies to look much brighter.
However, too much heat leads to another color change. Heat causes a magnesium atom at the center of each chlorophyll molecule to detach and be replaced by hydrogen atoms. This chemical change in chlorophyll molecules changes their bright green color to a dull gray-green.
Acids can also cause chlorophyll to lose a magnesium atom, and become that dull green. That is why it's a good idea to wait until serving to season your veggies with acidic dressings like vinegar or lemon juice.
You might be wondering if these high temperatures can affect more than color, like say nutritional content.
Well, high temperatures can break down most vitamins and antioxidants in vegetables, and hot water can draw vitamins and minerals out of plant cells. Also, though heat does destroy some of a vegetable's nutrients, it also makes others more easily absorbed. Our digestive systems can't break down many of the starches in plants. Cooking breaks these tough molecules down so our bodies can extract more nutrition from them.
For this reason, it is a good idea to include plenty of raw and cooked veggies in your diet.