Eosin methylene blue

Eosin methylene blue (EMB) is a selective stain for Gram-negative bacteria. It is a blend of two stains, eosin and methylene blue. A common application of this stain is in the preparation of EMB agar, a differential microbiological medium which inhibits the growth of Gram-positive bacteria and provides a color indicator distinguishing between those organisms that metabolize lactose versus those that do not.

On EMB if E.coli is grown it will give off a distinctive metallic green sheen (due to acid production). It is the only bacteria that reacts this way to EMB so it can be quickly used to identify E. coli

Reference
http://medic.med.uth.tmc.edu/path/emb.htm