This depends on whatyou're cooking:
Green vegetables: to keep a vibrant colour and "al dente" texture, you need to cook green vegetables (broccoli, green beans etc.) as quickly as possible. Over-cooking causes the plant's chlorophyll to leach into the water, making them dull and listless. Start cooking in boiling water and then "set" the colour by plunging into ice water when almost cooked (3-4 minutes). The vegetables can then be reheated for a minute before service without losing their colour.
Root vegetables: larger starchy vegetables, such as potatoes, need more time in a gentle heat so their cores warm up before the outsides overcook and disintegrate. These vegetables should be started in cold water and then brought to the boil.
