It's principle is lactose fermentation...Lactose fermenters metabolise the lactose in the media and produce acid byproducts,causing a color change in the colony.Strong acid production by organisms such as E.coli results in a metallic green sheen.Weaker fermentation of lactose results in colonies with a pinkish-purple color.Colonies of nonlactose fermenters remain colorless,or at least are no darker than the color of the media.Therefore,it shouldn't look yellow:-(