Final answer:
Gram-negative bacteria appear pink when dyed due to the thin peptidoglycan cell wall that allows the pink counterstain to be taken up by the cells.
Step-by-step explanation:
Gram-negative bacteria appear pink when dyed due to their thin peptidoglycan cell wall that does not prevent the crystal violet dye from washing away during the decolorizing step of the Gram stain procedure. This allows the pink counterstain, safranin, to be taken up by the cells, resulting in their pink color.