Final answer:
Gram-positive microorganisms stain purple due to their thick cell wall that traps the crystal violet dye used in Gram staining. A combination of Gram's iodine and alcohol in the staining process differentiates Gram-positive from Gram-negative bacteria, leading to the distinct purple and pink coloration respectively.
Step-by-step explanation:
Gram-positive microorganisms stain purple when the Gram staining technique is used. This staining occurs because Gram-positive bacteria have a thick peptidoglycan layer in their cell wall that traps the crystal violet dye used in the first step of the staining process.
During the staining procedure, Gram's iodine is added as a mordant to enhance the crystal violet staining by forming a complex with the dye, which is more difficult to wash out. Alcohol is then used as a decolorizer that dehydrates the thick cell wall of Gram-positive cells, further trapping the dye within. Gram-negative cells, having a thinner peptidoglycan wall and an outer membrane, do not retain the crystal violet and are decolorized by the alcohol, allowing them to be stained pink by the counterstain, safranin.