From Biosun FFI Wiki

Anthocyanin (derived from the Greek anthos: flower and kyáneos: purple) are water soluble vacuolar flavonoid pigments that reflect the red to blue range of the visible spectrum, depending on the pH of the surrounding solution. They are found exclusively in the plant kingdom, and have been observed to occur in all tissues of higher plants, providing color in everything from fruits to autum leaves. Grapes are the world’s largest fruit crop for processing colorant. Anthocyanins are composed of an aglycone (anthocyanidin), and also contain flavonoids. 

The most common commercial method of producing Grape Skin Extract involves the treatment of the skins with water containing sulfur dioxide or its equivalent. This increases extraction of the pigments as well as increases stability of the final product.

Anthocyanins will develop a purple to red-black color at a pH range of 3.0 to 5.5 and is ideal in a variety of beverages bases, carbonated drinks, and alcoholic beverages.