Final answer:
Gram-positive bacteria appear purple when dyed because of their thick cell wall, while Gram-negative bacteria appear pink because of their thin cell wall.
Step-by-step explanation:
Gram-positive bacteria appear purple when dyed because they have a thick cell wall without an outer membrane, which allows them to retain the crystal violet dye used in the Gram stain process.
On the other hand, Gram-negative bacteria appear pink because their thin cell wall does not prevent the crystal violet from washing away during the decolorizing step, and they take up the pink counterstain safranin.