How a acid or a alkali indicator indicates?

Acid-base indicators are substances that change color in response to the pH level of a solution. In acidic solutions, indicators often exhibit one color, while in alkaline solutions, they display a different color. This color change occurs due to the protonation or deprotonation of the indicator molecules, which alters their structure and light absorption properties. Common examples include litmus paper, which turns red in acids and blue in bases, and phenolphthalein, which is colorless in acidic solutions and pink in alkaline ones.