Final answer:
Gram-positive bacteria stain purple on Gram stain because of their thick peptidoglycan layer, while Gram-negative bacteria stain red or pink due to a thinner peptidoglycan layer and an outer membrane.
Step-by-step explanation:
Gram-positive bacteria stain purple upon Gram stain due to the presence of a thick peptidoglycan layer in the cell wall. The thickness of the peptidoglycan layer can range from 30-100 nm and is embedded with teichoic acids, which contribute to the rigidity of the cell wall and play a role in infection processes. In contrast, Gram-negative bacteria stain red or pink due to a thin peptidoglycan layer and the presence of an outer membrane containing lipopolysaccharides.