A substance that changes color when an acid or base is added is called an indicator. Indicators are typically weak acids or bases that undergo a change in color over a specific pH range. When an indicator is added to a solution, it will change color depending on the pH of the solution. For example, phenolphthalein is an indicator that is colorless in acidic solutions and turns pink in basic solutions. Litmus is another common indicator that turns red in acidic solutions and blue in basic solutions. Indicators are used in a variety of applications, such as titrations, to determine the pH of a solution.