Gram staining is a common technique used to differentiate Gram-positive and Gram-negative bacteria based on their different cell wall constituents. Colours of Gram-positive and Gram-negative bacteria on Gram staining respectively are _________.

The Gram stain technique distinguishes between Gram-positive and Gram-negative bacteria by colouring these cells purple or pink. Gram positive bacteria stains purple due to the presence of a thick layer of peptidoglycan in their cell walls, which retains the crystal violet these cells during the decolouring process. Gram negative bacteria stains pink, which is attributed to a thinner peptidoglycan wall, which does not retain the crystal violet during the decolouring process.