A technique or procedure for differentiating bacteria into Gram positive or Gram negative according to the different constituents in their cell walls. A thick peptidoglycan layer means the Gram positive bacteria will stain violet, while Gram negative bacteria stain red/pink as a result of a thinner peptidoglycan layer. It involves three processes, i.e, staining (with crystal violet), decolorization, and counterstaining, usually with safranin.