what is the compound involved in changing of blue litmus to red litmus paper in acid

An acid is defined as a substance that turns blue litmus paper to red,while base is a substance that changes red litmus paper to blue.Litmus paper is an filter paper which is treated with a natural water-soluble dye obtained from lichens which is used in laboratories to test the substances as acid or base or neutral.This litmus is an indicator made from lichens.It is a water-soluble mixture of different dye types extracted from lichens.Such plants occurs in several varieties.For ex- one variety of lichen called Rocella Tincotoria when allowed to react with substances like ammonia, lime etc. it gives a blue-color material,which is used as the coloring matter for paper,which is known as blue litmus paper and used to test acids.Another variety Orchil or cudbear is a red dye obtained from another species of lichen used to make red litmus paper which is used to test bases.