In theory, any substance that undergoes a reversible chemical change when pH changes can be used as an acid-base indicator. In practice, a sharp change in some easily detectable property of the substance is required. Notice that most of the materials are derived from plants. Almost any flower, fruit, or plant part that is red, blue, or purple contains a class of chemical compounds called anthocyanins that change color with pH. The color of a flower or fruit depends on which anthocyanins are present, the pH of the pigment-bearing tissues, and the presence of other pigments, like yellow flavones. To learn more about why indicator molecules change color with pH, see Water to Wine .
Beets change from red to purplish in very basic solution.
Blackberries, black currants, and black raspberries change from red in acids to dark blue or violet in basic solution. Blue and red grapes contain several different pH-sensitive anthocyanins. For example, blue grapes are colored by a monoglucoside of malvinidin that changes from deep red in acidic solutions to violet in basic solution. Red wines naturally contain these same pigments. Carrots Cherries and cherry juice is bright red in acidic solution but purple to blue in basic solution.