mar·ti·us yellow \ˈmärsh(ē)əs-\noun Etymology: after Karl A. Martius died 1920 German chemist 1.usually capitalized M, often capitalized Y: a yellow dye that is a salt of a dinitro-alpha-naphthol and is used chiefly as a biological stain 2.often capitalized M: a light greenish yellow that is redder and paler than sulphur yellow (sense 2)