What color does an acid turn litmus paper?

Acids turn litmus paper red. This change in color occurs because litmus paper is a type of pH indicator that reacts to the acidity or basicity of a solution. When an acid is present, it donates protons (H⁺ ions) to the solution, lowering the pH. The presence of these free protons alters the chemical composition of the dye in the litmus paper, causing it to display a red color. In contrast, bases would turn litmus paper blue, which is why blue and the other options do not represent the behavior of acetic acid with litmus paper.