石蕊是一种蓝色色素，分子式C₇H₇O₄N，常用作酸碱指示剂（pH指示剂）。是石蕊试纸的原料。

• 蓝色粉末，部分溶于水和乙醇。

Erythrolitmin (also called erythrolein) is the active ingredient extracted from the Litmus lichen, used in chemistry as a pH indicator. The erythrolitmin molecule is related to the orceins, and consists essentially of several phenoxazone and orcinol residues.