In pH indicators, what color typically indicates an acid?

In pH indicators, the color red is commonly associated with acidic solutions. Many pH indicators, such as litmus paper, turn red when exposed to an acidic environment, typically with a pH level below 7. This color change occurs because the pH indicator undergoes a chemical reaction that alters its structure in response to the concentration of hydrogen ions (H+) in the solution. An increase in these ions lowers the pH, resulting in the red coloration, which denotes acidity.

The other colors listed do not indicate acidity; for example, green is often neutral (around pH 7), while blue and yellow may represent different pH ranges, typically indicating either alkaline conditions or a weak acid, respectively. Therefore, red is the definitive color indicating an acidic nature.