After Gram's iodine is added, what color do the cells appear under a light microscope?

When Gram's iodine is added to cells during the Gram staining process, it acts as a mordant, enhancing the primary stain (crystal violet) that is applied before it. Under a light microscope, cells that are positively stained will appear purple or violet due to the retention of the crystal violet stain. This is typical of Gram-positive bacteria, which have a thick peptidoglycan layer in their cell walls that holds onto the dye even after decolorization.

In contrast, Gram-negative bacteria, which have a thinner peptidoglycan layer and an outer membrane, will not retain the crystal violet during the decolorization step and will appear pink or red after the application of the counterstain (safranin).

In summary, after the addition of Gram's iodine, Gram-positive cells typically appear purple, while Gram-negative cells will not retain that color and will appear pink after the complete staining process.