Final answer:
The acid dye used as the secondary step in Gram staining, after using a decolorizing agent, is Safranin, which stains decolorized cells pink.
Step-by-step explanation:
The acid dye usually used as the second step in the staining process is Eosin. However, in the context of Gram staining, which is a common technique used to differentiate bacterial species into two large groups (Gram-positive and Gram-negative), the secondary counterstain that is used after the decolorization step is Safranin. After the primary stain of crystal violet and the decolorizing step, Gram-positive cells retain the violet color, whereas Gram-negative cells are decolorized. The addition of Safranin stains these decolorized, Gram-negative cells pink, allowing them to be distinguished from the purple Gram-positive cells under a microscope.