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. It does so by colouring a portion of a specimen (e.g. of a tissue or of a cell) that remained uncoloured following the first staining using a dye of different color.
For instance, in Gram staining, two dyes are used: the Crystal Violet and Safranin. The latter is the counterstain or contrast stain used in this method. The Gram staining is used to distinguish Gram positive bacteria from Gram negative bacteria.