Eosine methylene blue (EMB ) agar is a differential medium for coliform bacteria.  Coliforms (most commonly E. coli), which are lactose fermenters,  form blue-black colonies with a ‘greenish metallic sheen’.  Nonlactose fermenting Gram-negative bacteria will form colonies with a variety of colors, from clear to pink or amber.  The medium is slightly inhibitory to Gram-positive bacteria, some of which may form small, pin-point colonies.