The color change in red cabbage juice indicates the pH level of the substance it comes into contact with. Red cabbage contains anthocyanins, which are pH-sensitive compounds that change color in response to the acidity or alkalinity of a solution. If the juice turns pink or red, the substance is acidic (pH less than 7). If it turns green or yellow, it’s basic (pH greater than 7). A purple color indicates that the substance is neutral (pH around 7).