Gram-positive bacteria appear purple or blue after gram staining. Gram-positive bacteria have a thick peptidoglycan layer in their cell walls, which retains the crystal violet dye during the gram staining procedure. The retained dye gives the bacterial cells a purple or blue color when viewed under the microscope. Gram-negative bacteria, on the other hand, have a thinner peptidoglycan layer and an outer membrane made of lipopolysaccharides, which does not retain the crystal violet dye. These bacteria appear pink or red when counterstained with safranin, a counterstain used in the gram staining procedure.