Staining Bacteria?

Question

For differential staning, how does a counterstain differ from a primary stain?

Answer 1

A counterstain stains those bacteria that are negative in the primary stain. It adds more contrast and makes every cell visible.

Answer 2

The stain itself isn't different except in color really.  You use a binding agent with the first stain, cause it's usually Gram positive or Acid fast so needs something extra to hold the dye.  Then after rinsing, you apply the secondary dye to dye the negative cells, which hold that dye, for contrast.