thebioguru:

Gram staining is a method used to differentiate between cell wall characteristics by using a safranin stain that adheres to peptidoglycan (a sugar/amino acid polymer that forms bacterial cell walls).

Gram-positive bacteria possess a thick layer of peptidoglycan exterior to the plasma membrane that retains the dye crystal violet. They appear to be a dark purple or blue/black.

Gram-negative bacteria have a cell wall composed of a thin layer of
peptidoglycan and lipids that is sandwiched between two plasma membranes. Because of this, the crystal violet washes out easily and they become stained a light red or pink color with safranin stain.