Answer:
Both the stain which is used in gram staining may be used to do simple stains on gram-positive and gram-negative cells but gram-positive bacteria end beings stained violet and gram-negative being stained pink because of the composition of their cell wall.
As the gram-positive bacteria have an outer thick layer of peptidoglycan it is not decolorized after destaining step and the mordant iodine also helps it to do that but as the gram-negative bacteria have outer LPS layer in cell wall so it get decolorized after destaining as alcohol in destaining step degrades some LPS layer.
So after counter staining the gram-negative bacteria appears pink and gram-negative bacteria retain crystal violet so they appear purple.
