Why did the grape juice change color when an acid or base was added?

The color change in Grape Juice when an acid or base is added is due to the presence of anthocyanins, which are pigments found in the juice. These pigments can change their structure depending on the pH of the solution; in acidic conditions, they often appear red, while in basic conditions, they can turn blue or green. This reaction highlights the sensitivity of anthocyanins to pH changes, making grape juice a useful natural pH indicator.