Gram staining uses two dyes : Crystal violet and Fuchsin or Safranin ( the counterstain ) to differentiate between Gram-positive bacteria ( large Peptidoglycan layer on outer surface of cell ) and Gram-negative bacteria.
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Other compounds used to color tissue sections include safranin, Oil Red O, Congo red, Fast green FCF, silver salts, and numerous natural and artificial dyes that usually originated from the development of dyes for the textile industry.
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Gram-positive bacteria retain the crystal violet dye, and thus are stained violet, while the Gram-negative bacteria do not; after washing, a counterstain is added ( commonly safranin or fuchsine ) that will stain these Gram-negative bacteria a pink color.
