The dye or stain that is used to differentiate one component or cellular structure from another, or to differentiate an entity from another in a specimen.

What is a counterstain used for?

A counterstain introduces color to specific cellular structures to provide contrast to the colored enzyme substrate. Counterstaining aids in visualization and target localization, facilitating interpretation of morphology and cell structure within the tissue section.

What's the purpose of counterstain in Gram staining?

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.