Litmus paper is one of the quickest ways to test a liquid for its acidity or basisity. Dyes called indicators show very quickly if the water is acidic or alkaline. Litmus is one such dye. Litmus paper turns red if the solution is acidic and it turns blue if the solution is alkaline.
Some vegetables like red cabbage and beetroot have this dye and they change colour during cooking. If the tap water is hard or alkaline, the vegetables will acquire a deep purplish blue colour.
Below Litmus Paper info from Wikipedia (Read More):-
Litmus is a water-soluble mixture of different dyes extracted from lichens, especially Roccella tinctoria. The mixture has CAS number 1393-92-6. It is often absorbed onto filter paper. The resulting piece of paper or solution with water becomes a pH indicator (one of the oldest), used to test materials for acidity. Blue litmus paper turns red under acidic conditions and red litmus paper turns blue under basic (i.e. alkaline) conditions,the color change occurring over the pH range 4.5-8.3 (at 25°C). Neutral litmus paper is purple in colour.[1] The mixture contains 10 to 15 different dyes (erythrolitmin (or erythrolein), azolitmin, spaniolitmin, leucoorcein and leucazolitmin). Pure azolitmin does show nearly the same effect as litmus.
http://en.wikipedia.org/wiki/Litmus_paper
