In the flavonoid pigment family, the blue, red and purple hues (specifically anthocyanin pigments) of certain fruits and vegetables such as grapes, blueberries and red cabbage, can react with metals such as cast iron, tin, and sometimes aluminum. These metals will make these pigments turn various shades of blue.

To prevent discoloration, cook in non-reactive cookware such as stainless steel. Also, store these types of foods in stainless-steel or glass containers to maintain their vibrant colors.