Litmus paper changes colors in response to an acidic or base solution. Acidic solutions result in a color change that is red. Neutral solutions yield a purple color.
Solutions with pH levels below seven are considered acidic. Those with pH levels at seven are considered neutral. Litmus paper is paper that has been treated with a mixture of natural dyes known as lichens, or Roccella tinctoria.
Litmus paper is not the only method of using color change to detect pH level. One natural way to do so is to use red cabbage juice. It appears red in acidic solutions, blue in neutral ones and greenish-yellow is base solution with pH levels at 12.