Generally do not change the color of red or blue litmus paper, though dedicated purple litmus paper exists for this purpose. Types of Litmus Paper
A more versatile option that can turn red in the presence of an acid or blue in the presence of a base. Testing Procedures litmus paper
Used for testing the acidity of soil, water, household chemicals (like vinegar or baking soda), and even in medical settings to assess body fluid pH. Generally do not change the color of red