A counterstain is a stain with color contrasting to the principal stain, making the stained structure more easily visible.

An example is the malachite green counterstain to the fuchsine stain in the Gimenez staining technique.

Another example is eosin counterstain to haematoxylin in the H&E stain.

Also in Gram staining, crystal violet stains only Gram-positive bacteria, and safranin counterstain is applied which stains all cells, allowing the identification of Gram-negative bacteria as well.

Staining