A little background for those of you who aren't enthralled by the whole heirloom/variety produce thing: Carrots naturally come in many colors, and the orange varieties we eat are descended from ones bred by 16th-century Dutch horticulturalists intent on impressing the ruling House of Orange.
I certainly fall into the "why buy a regular tomato when you can buy a Hungarian green zebra tomato?" camp. I've bought purple carrots before for snacking—they have a nice herbaceous snap to them—but I picked up a bunch at the Ballard Farmers Market and decided that this time I would cook with them. Also in the basket: some frilly, delicate Russian kale. Carrots + kale + first day of fall = white-bean stew with greens. So I sauteed up my carrots with onions and celery, then added in chicken stock, beans, ham hock, etc.
No one had warned me that the anthocyanins in the purple carrots would turn the broth a purplish brown.
It looked, well, not natural. I looked around for a tomato to see if that would at least tint it red, but all I had was a black brandywine—also purple, and not as attractive as the photo in the link—plus some tomato paste. The combination of the two now gave the broth a murky reddish-purple color, and the beans, as they cooked, took on a purple-gray cast. Finally, I dumped in the kale, which flashed bright green before fading to black in about five minutes.
I once cooked an all-black-food spread for a Day of the Dead party, but I've never aimed for grayish purple with black stripes. Soup tastes fine, but instead of bringing some in for lunch, I think I'm going to have to eat it at home. By candlelight. A victim, once again, to my own pretensions.