Vegetables – How to steam vegetables

Typically, vegetables will lose their colour if they are over-cooked, so it's probably worthwhile 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; orange vegetables contain keratin. Each of these pigments requires a different approach when cooking to preserve colour.

When cooking green vegetables, never add an acid, such as lemon juice or vinegar, as this will cause the vegetables to discolour quickly. Always cook green vegetables rapidly and preferably without a lid.

With red and white vegetables, which contain flavonoids, the converse to green vegetables is true. 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 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.

To prevent the vegetable from going 'off' in the freezer.

From answers.com:

Blanching is the scalding of vegetables in boiling water or steam. Blanching slows or stops the action of enzymes. Up until harvest time, enzymes cause vegetables to grow and mature. If vegetables are not blanched, or blanching is not long enough, the enzymes continue to be active during frozen storage causing off-colours, off-flavours and toughening. Blanching time is crucial and varies with the vegetable and size of the pieces to be frozen. Under-blanching speeds up the activity of enzymes and is worse than no blanching. Over-blanching causes loss of flavour, colour, vitamins and minerals.