In the Gram stain procedure, gram-positive bacteria appear purple because their thick peptidoglycan cell wall retains the primary crystal violet stain during the decolorization step, dominating the pink color of the safranin counterstain. Conversely, gram-negative bacteria appear light red or pink because their thin peptidoglycan cell wall allows the crystal violet to wash away during decolorization, leaving them to be stained by the safranin counterstain.