Why is counter-staining necessary when using a differential staining technique such as the gram staining?

Answer 1

counter staining is necessary because the first stain may not have bound to the organism.
ex: if the crystal violet in the gram staining procedure doesn't bind to an organism after the mordant is added and the excess is washed off, you wouldn't be able to see the cells which didn't pick up the crystal violet. therefore, you need the counter stain to be able to see these gram negative organisms.