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

Litmus is a natural dye made from small plants called, "lichens". These plants are of several varieties and grow in abundance in the Netherlands. When lichen called Rocella Tincotoria is allowed to react with ammonia, potassium carbonate and lime it gives a blue-color material, which is used as the coloring matter for paper. The paper is dipped into it and dried. This is what is known as blue litmus paper and used to test acids.

Orchil or cudbear is a red dye obtained from another species of lichens. This is used to make red litmus paper Alkannet or alkanna is another dye obtained from root of the plant Alkanna Tinctoria. The coloring ingredient, alkannin is soluble in alcohol, benzene and ether. When white paper is impregnated with an alcohol solution of alkannet, it becomes red. This paper is turned to blue shade by alkalis.