It loses its violet color after a
Gram stain because the violet does not get into the cells.
It possesses Gram negative cell wall, which has an extra layer of peptidoglycan surrounded by a lipopolysaccharide outer membrane. The Gram negative bacteria tends to be harder to kill because of that extra layer, and during staining, the purple coloring does not get through the membrane, leaving only Gram positive bacteria purple under the view of the microscope.