Methylene green is not commonly used in medical histotechnology. It stains nuclei green, and is sometimes used as a counterstain to red or purple primary stains.
The formula on the left is from Aldrich, that on the right from Conn.
R. D. Lillie.
Conn's Biological Stains
Williams & Wilkins, Baltimore, MD., U.S.A.
Aldrich chemical catalogue, 1992
Aldrich Chemical Company, Milwaukee, WI, USA.