The cell envelope contains our cell wall, which is made of a material known as a peptidoglycan, and it also contains the inner plasma membrane. In some cases, we also have an outer plasma membrane. Basically, our grampositive bacteria contain a very thick peptidoglycan cell wall, and that allows the purple dye to remain inside that cell. And that's exactly why on the microscope, these types of bacterial cells appear purple. But because of the thin cell wall of our gram negative, that purple dye can be easily washed off. And so these gram negative bacterial cells appear pink under our microscope.