Anthocyanin pigments are responsible for the bright red of a really ripe strawberry, the crimson color of a cherry, or the deep blue-black of plums.
Anthocyanin pigments are sensitive to the acid-alkaline balance of recipes. When a recipe becomes more alkaline, these pigments will appear more of a blue color. As the dish is made more acidic, it will become more reddish-purple.
Keeping it Red
Using acidic ingredients in recipes with purple/red/blue fruits and vegetables will keep fruits and vegetables a bright red.
When baking with berries like blueberries and raspberries, use sour cream or buttermilk in the mix. The acidity of the dairy will help keep the berries from turning a funky bluish green-black.
Berries add a bright pop of color in salads that stay bright because of the acid typically used in dressing.
Cooking red cabbage with acidic ingredients like red wine vinegar will keep it from turning blue.
Avoid cooking in aluminum and iron pans, trace metals will cause foods to turn blue, the same as Alkaline environments .