Litmus paper is a type of pH indicator that changes color in the presence of acids or bases. Acids will turn blue litmus paper red. This is because acids have a high concentration of hydrogen ions (H+), which react with the litmus dye to form a red compound. Bases, on the other hand, have a high concentration of hydroxide ions (OH-), which react with the litmus dye to form a blue compound. Therefore, acids will turn litmus paper red, while bases will turn litmus paper blue.