Answer:
The contrasting color is important in gram staining because through contrasting color we are able to distinguish between gram-positive bacteria and gram-negative bacteria.
First crystal violet is used which stain all the cells violet. Then iodine solution is added which forms a complex with the crystal violet. After this acetone is used to destain the cells.
At this step, gram-negative cells become detained because acetone dissolves the lipopolysaccharide cell wall of gram-negative bacteria that results in leakage of crystal violet from the cell.
Gram-positive cell wall does not get detained because its cell wall is made up of peptidoglycan which does not dissolve in acetone so it remains purple due to the presence of crystal violet in the cell wall.
After this a counter strain safranin is used which stain destained gram-negative cells pink, therefore, gram-negative cells are seen pink due to safranin under microscope.
Therefore purple cells indicate gram-positive cells and pink cells indicates gram-negative cells.
