The Blue Flower

• The anthocyanins are a part of the larger group of phenolic compounds, which give fruits and vegetables much of their color, flavor and aroma. The anthocyanins get their name from a pair of Greek words meaning "blue flower," and they're responsible for colors of many red and purple vegetables, from red onions to purple cabbage and blue potatoes. The pigments in many of these vegetables occur throughout their cells, but in some -- including purple varieties of beans and peppers, as well as the tips of asparagus -- they're more ephemeral.

Beauty Is Skin Deep

• In purple peppers, as with beans and asparagus, the purple-hued anthocyanins are confined to a thin layer of cells near the plant's surface. Like a thin layer of cloud over a massive forest, their own color serves only to hide the green underneath. When you begin to cook the peppers, the cells immediately under the surface swell and burst in the heat. This releases moisture from within the cells, diluting the purple tint. Anthocyanins are water-soluble, so they quickly spread throughout the juices of the cooked pepper and fade to invisibility.

Muddying the Waters

• The water-solubility of the anthocyanins in purple peppers and similar vegetables can pose a second difficulty. If you cook them in water, or a water-based sauce, some of the purple pigment will leach out into the cooking liquid. This can lend a strangely drab appearance to your other ingredients, and sometimes gives the sauce a disconcertingly dark appearance. Anthocyanins can also change color unpredictably in hard water, or if they're cooked in aluminum cookware. If you've added a pinch of soda to your cooking water, to preserve the vivid green of other vegetables, it will react with the anthocyanins to create a peculiarly unpleasant shade of blue.

Minimize the Damage

• There's no effective way to cook purple peppers and preserve their color. The best you can do is stir-fry them lightly in hot oil until they're just warmed, and their color only slightly changed. You can minimize their tendency to discolor other foods. Use non-reactive stainless steel, enamel or glass cookware whenever possible, rather than aluminum. Incorporate acidic ingredients, such as lemon juice, wine vinegar or tomatoes, into your vegetable dishes to reduce the risk of discoloring the sauce or remaining vegetables. Even if the sauce is already slightly discolored, adding a dash of an acidic ingredient will return its hue to a more natural appearance.