A compound that changes color when in contact with an acid or base is called an indicator. Indicators are substances that undergo a visible change in color when the acidity or basicity of a solution changes. They are often used to determine the endpoint of a titration, which is the point at which the reaction between the acid and base is complete.

There are many different types of indicators, each with its own unique color change. Some common indicators include:

* Phenolphthalein: This indicator is colorless in acidic solutions and turns pink in basic solutions.

* Methyl orange: This indicator is red in acidic solutions and turns yellow in basic solutions.

* Litmus: This indicator is red in acidic solutions and blue in basic solutions.

Indicators are an important tool in chemistry, as they allow us to easily and accurately determine the acidity or basicity of a solution.