Put the following substances in the order they are used in a Gram stain:
Counterstain
Decolorizing agent
Mordant
Primary stain

The primary stain is the first dye applied in the Gram staining process, typically crystal violet. It penetrates all bacterial cells, coloring them purple initially, which sets the stage for differentiating bacteria based on their cell wall properties.

The mordant, usually iodine, is applied after the primary stain to form a complex with the dye inside the cells. This complex helps fix the primary stain, making it less soluble and enhancing the retention of the dye in Gram-positive bacteria.

The decolorizing agent, often alcohol or acetone, selectively removes the primary stain from Gram-negative bacteria due to their thinner peptidoglycan layer. After decolorization, the counterstain (commonly safranin) is applied to stain the now colorless Gram-negative bacteria pink/red, completing the differentiation.