Gram Positive Bacteria (Cocci). Gram-positive bacteria are those that retain a crystal violet dye during the Gram stain process. Gram-positive bacteria appear blue or violet under a microscope, while Gram-negative bacteria appear red or pink. The Gram classification system is empirical, and largely based on differences in cell wall structure. The purpose of Gram staining is to visually differentiate groups of bacteria, primarily for identification. Cocci bacteria are round in shape. Magnification x400, 35mm film.