When I steam green vegetables, like broccoli, they come out bright green. A few minutes later though, and they have darkened and dulled. Is this a symptom of cooking them for too long or is there some other way to preserve the bright green color?
Typically, vegetables will loose their colour if they are over-cooked, so it's probably worth while cooking them for a shorter period of time.
Usually steaming is a great way to preserve the nutrients and colour of vegetables, as is stir-frying rapidly.
Different kinds of vegetables contain various pigments in their skins. Green vegetables contain chlorophyll; red and white vegetables contain flavonoids and orange vegetables contain keratin. Each of these pigments requires a different approach when cooking to preserve colour.
When cooking green vegetables, never add a acid, such as lemon juice or vinegar as this will cause the vegetables to discolour quickly. Always cook green vegetables rapidly and preferably with out a lid.
With red and white vegetables, which contain flavonoids, the converse to green vegetables is true when cooking these. Adding an acid during the cooking process will retain and even restore the colour. For red vegetables use a red or white wind vinegar. For white vegetables use a slice of lemon.
Orange vegetables, which contain keratin and are generally fairly robust. For the most part they can be cooked with or without the addition of an acid and they will usually retain their colour.
Another way to preserve the colour is to blanch the vegetable in boiling water for a minute or so, then plunge them into ice water. Doing this stops the cooking process instantly. Using the method is useful if you intend to freeze vegetables.
you need a big pot of water to blanch small amount of vegetable, small portion at a time.
thus, use large amount of water and small amount of vegetable, then stop the cooking by quenching the reaction using ice cold water:
that's basically it