Why does red cabbage sometimes turn purplish-blue when cut with a knife?

If a non stainless metal knife is used to cut a red cabbage, the anthocyanin pigment in the cabbage will react with the metal ions in iron, tin, or aluminium and create off-colors. Stainless steel and glass are consequently the best choices for the preperation and storage of foods rich in anthocyanins. Most of red, purple, and blue color seen in vegetable and fruits derive the color from anthocyanin.