Methylene blue, chemically designated as methylthioninium chloride, is a heterocyclic aromatic molecule that exhibits a number of chemical and physical properties that enable its use as a colorant.

One of the most important aspects of methylene blue is its ability to provide color based on its structure as a phenothiazine derivative. This structure allows for extensive conjugation of π-electrons, which leads to the intense blue color of the molecule.

Chemically, methylene blue has the ability to act in redox reactions. It can switch between a reduced, colorless form and an oxidized, blue form. This is particularly useful in biological systems, as it can indicate changes in the redox status of a cell or tissue in this way.

In addition, methylene blue has a high affinity for certain biological molecules, particularly nucleic acids. By interacting with the phosphate groups of nucleic acids, it enables selective staining of cells and tissues.