I bought some beautiful purple kohlrabi at the Farmers' Market last Saturday, only to find that most people have never heard of it, which is a shame. It is not only purple, it is delicious and generous: you can cook the greens like kale, eat the bulb in salad (like radish-meets-apple), or cut the bulb into chunks and roast it like any roasted winter veggie.
Here (above), I've peeled a bulb (the purple part is only on the outside-- the inside is light green) and julienned the flesh, then mixed it with purple savoy cabbage and red radishes, dressed with red wine vinegar and Bariani olive oil, for a quick salad.
Here, I heated some olive oil in a pan and sauteed some sliced garlic and a couple anchovies; then I added the kohlrabi greens (remove the stems, as you would with kale) and sauteed until bright green (about 5 minutes). These would be delicious plain, but I had some leftover pasta, so I put that in as well, with a little water, and steamed it all until the flavors had combined.