There are lots of misnomers in the world of plant food colors. “Red” cabbage and onions, and lettuce definitely look purple, purple potatoes can look blue. (And cooked blueberries are definitely purple.) Lots of “black” fruit and grains turn out purple: black rice, black mission figs, black grapes, blackberries. Perhaps I ought to forgive whoever named all of these plants, though. Purple is very much in the same family of the red/blue anthocyanins. And that means purple plant foods get their coloring from anthocyanins, which are very shifty, pH sensitive colorants. So most purple plant foods have the same properties for the cook as those blue and red anthocyanins: stable when exposed to heat, but sensitive to pH. It is worth noting that many anthocyanin-rich purple foods often also have a high concentration of tannins. Tannins produce the sensation (rather than a taste) of astringency. Think of the dry, puckering sensation you get from a badly made cup of tea, and you know what astringency is. That’s why red wine (processed with the purple/red skins) has more tannins than white wine and purple lettuces are more astringent than green ones. In moderation, astringency can be good, but too much and it might make you pucker. If your plant foods are overly astringent; dairy, sugar and oil will tone down the sensation while salt and acid will magnify it.
Violets: Violet flowers are edible (and gorgeous) fresh. Their color and aroma can vary greatly. I find that most of the wild violets in this part of the world (northeastern US) do not have very much aroma of flavor. So… suitable as a garnish. More fragrant violets are used to make a floral and bright purple liqueur, Creme de Violette. Try adding fresh violets to a dessert plate, or creme de violette to a fancy cocktail.
Lavender: Lavender has a potent aroma and flavor. So much so, in fact, that I would never use enough lavender to color a dish. A little lavender is good, more might make you think you’re eating a fragrant bar of soap. Try infusing a little lavender in sweet, creamy dishes like panna cotta or ice cream or making a lavender syrup to sweeten lemonade. Or you can use it in savory dishes, with a quick infusion at the end of cooking, just like you would use rosemary.
Chives: Chive flowers are an incredible natural garnish. They look rather like a bright purple sea urchin. The oniony flavor is more mild than in the stems of chives, but it is still pleasant. Add chive blossoms whole as a garnish to salads, or cold dishes.
Grains & Beans: Many grains and beans have heirloom purple varieties (rice, corn, quinoa, black soy, black turtle beans) . The purple varieties tend to be a little darker and stronger in flavor. Some of these varieties have incredibly high concentrations of anthocyanins. Black soy beans have very high levels of anthocyanins, but the color of soy products (soy milk and tofu) made from these beans is only slightly affected. Forbidden rice (not the purple rice pictured) has so much pigment it looks black when dry, but when cooked it looks (and turns the cooking water) deep purple. Try using a dark, purple grain as a base for a meal it can make a striking backdrop, especially when you are serving along with brightly colored vegetables.
Greens: Among greens, there are many varieties that have purple varieties. Purple-leaved lettuces, kale, purple cauliflower and of course, purple cabbage. When eaten raw, these purple varieties tend to have more tannins than their greener counterparts. So if you are making, say, a red leaf lettuce salad you might want to serve it with a slightly sweet or creamy dressing, rather than an acid-based vinaigrette. As you might remember from the discussion of blue foods, red cabbage can be boiled and concentrated to make a food dye, with a basic pH this dye is blue (see image below of dye and leaves). But with a slightly acidic pH, the dye will be an intense purple. You can add this purple dye to frosting or acidic cake mixes for color. (Though I’d probably just add a berry puree.)
Potatoes & Sweet Potatoes: Both potatoes and sweet potatoes have purple varieties. Boiling the potatoes will push the hue toward the blue end of the spectrum, while baking or roasting them will keep them closer to the raw purple color. While intensely colored with anthocyanins, these potatoes do not tend to bleed their color too easily, and it is difficult to damage their color, even with acidic dressings. All this makes them an ideal candidate for a colorful ingredient swap. Try swapping purple potatoes next time you make a potato salad or roasted potatoes.
Red Onions & Shallots: Both red onions and shallots have a purplish hue when they are raw. Shallots tend to lose their pale hue when they are cooked. Red onions will become less vibrant, but still remain purplish. If red onions are pickled, the purple color (unstable) will turn bright magenta. Try adding raw, sliced onions to an otherwise green salad. If the flavor of raw onions is too strong for you, soak the onions in cold water to make them milder. Still too strong? Boil the onions for a minute, then shock in cold water.
Grapes, Figs & Stone Fruit: The pigment from grapes and stone fruits (plums, apricots) is concentrated at the skin. Always use these fruits when they are ripe to avoid an overly tannic sensation. Since the pigment is concentrated at the skin, the color is best displayed when the fruit is whole, or has been cooked with the skin. But cooking fruit with the skin can be tricky– remember how blueberries turn purple when cooked? Well, some purple fruits can turn red for the same reason. Exposure of the pigments in the skin to the acid in the fruit makes the unstable anthocyanins slip into the red category. Think of red wine that comes from purple grapes. So if you want a deep purple color for a fruit dish chose the darkest purples- concord or black grapes, black mission figs, prunes, dried currants and raisins.
Berries: Many berries have incredible purple pigments: blackberries, blueberries, huckleberries even the trendy acai berries. All of these dark berries will have incredible purple color when served either fresh or cooked.
Walnuts: Walnuts are the really weird one in this list. If you bake a bread with lots of walnuts in it, the bread will turn purple. Whether or not you want that purple is a matter of taste (I know fine bakers who have differing opinions on this matter) The purple color comes from the papery skin on the outside of the walnut reacting* with the dough. If you want to avoid purple bread, then toast the walnuts first and when they’ve cooled enough to handle, rub the skins off and discard. Conversely, if you want an even purple tone, you can remove these skins and make sure that they are well mixed into your dough. I’ve also read (though I haven’t experienced this one first hand) that a walnut infused cream sauce will turn green herbs purple.
*As much research as I did, I could not find out exactly what chemical reaction is happening. I just know it is a compound from the skin of the walnut. If you can enlighten me I would be very, very grateful. I might even bake you some walnut bread.