Litmus paper is red, litmus paper is blue, put these papers in a liquid or a gas and its hydrogen ion concentration will shine true. Litmus paper, or pH paper, is a tool that scientists use to discover whether a liquid or gas is an acid or a base.You can use litmus paper to see whether bleach is an acid or a base by examining what color the paper turns.n the other hand, a blue piece of litmus paper put in an acid will turn red. If you have a gas or a liquid with a neutral pH level, neither the red nor blue litmus paper will change color when introduced to the substance in question.