The most common example of differential staining is Gram staining. Gram staining separates bacterial cells into two groups -  gram positive and gram negative. The microbial cells that keep a primary stain color (crystal violet) are stained purple and labeled as gram-positive. The microbial cells that do not keep the crystal violet color are counterstained red with safranin, and are labeled gram-negative. There are several morphological differences between gram-positive and gram-negative cells. Gram positive microbial cells do not have lipid-rich cell walls. In contrast, gram-negative cells have a higher lipid content in cell walls. During decolorization, ethanol may extract the lipids, increasing the permeability of cell walls. Therefore, the crystal violet-iodine complex easily disintegrates. In contrast, gram-positive bacterial cell walls become dehydrated, decreasing the porosity, and the crystal violet-iodine complex is retained.