Explanation – Gram staining is a differential staining technique that separates bacteria into two groups based on the differences in their cell wall structure. Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet stain used in the staining process, giving them a purple color under the microscope. In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane that prevents the crystal violet stain from penetrating, causing them to appear red or pink after counterstaining with safranin

Source: Forum IAS