Microbiology Gram's method
What colouring agents are used to stain Gram-positive and Gram-negative bacteria?
Gentian violet or methyl violet for G +, fuchsin for G-
According to Gram, bacteria are stained with aniline dyes - gentian or methyl violet (as @argentum already noted), then the dye is fixed with an iodine solution. Upon subsequent washing of the colored preparation with alcohol, G + are firmly stained in blue, in contrast to gram-negative ones, which become discolored when washed. After washing with solvent for Gram staining, a contrast red dye is added, which stains all Gram-negative bacteria red or pink.
Gram-positive bacteria have a thick mesh-like cell wall made of peptidoglycan (50–90% of cell envelope), and as a result are stained purple by crystal violet, whereas gram-negative bacteria have a thinner layer (10% of cell envelope), so do not retain the purple stain and are counter-stained pink by safranin.