You're describing a type of dye called an acid-base indicator. These dyes change color depending on the pH of the solution they're in.

Here's how it works:

* Acidic solution: The dye molecule exists in a specific form that absorbs certain wavelengths of light, resulting in a particular color.

* Basic solution: The dye molecule changes its structure due to the reaction with hydroxide ions (OH-). This altered structure absorbs different wavelengths of light, leading to a different color.

Common examples of acid-base indicators:

* Phenolphthalein: Colorless in acidic solutions, pink in basic solutions.

* Litmus paper: Red in acidic solutions, blue in basic solutions.

* Methyl orange: Red in acidic solutions, yellow in basic solutions.

* Bromothymol blue: Yellow in acidic solutions, blue in basic solutions.

These indicators are widely used in chemistry to determine the pH of a solution and to monitor the progress of acid-base reactions.