Methylene blue (methylthioninium chloride) is a synthetic basic dye. It occurs as dark green crystals or powder with a bronze-like lustre, and its solutions in water or alcohol are deep blue. It is used to dye paper and office supplies, to tone silk colours and as a textile and leather dye. It is also used as a biological and bacteriological stain, as an indicator and as a reagent in oxidation-reduction titrations.