Counter Staining with Safranin
Safranin is retained, very much like Crystal violet, by the peptidoglycans in the cell wall. However, the main difference is the thickness of the peptidoglycan layers in Gram positive and Gram negative bacteria.

Gram positive bacteria, due to their thicker peptidoglycan layer, have a higher affinity to and retention rate of crystal violet. On the other hand, Gram negative bacteria, due to a thinner peptidoglycan layer cannot retain crystal violet as well and are more likely to retain safranin.