Gram negative bacteria that ferment the lactose produce acid which turns the colonies dark purple as the acid acts upon the dyes. In addition, certain lactose-fermenting bacteria produce flat, dark colonies with a green metallic sheen. Other lactose fermenters produce larger, mucoid colonies, often purple only in their center. In EMB agar, E.coli colonies have a characteristic green sheen. Lactose non-fermenters are either colorless or light lavender