Microbiology

Purple/blue, because they retain the crystal violet-iodine complex in their thick peptidoglycan wall.

Pink/red, because their thin peptidoglycan layer loses crystal violet during decolorization and takes up the safranin counterstain.

Crystal violet (primary stain), Gram's iodine (mordant), alcohol/acetone (decolorizer), safranin (counterstain).

Decolorization with alcohol/acetone; over-decolorizing makes gram-positives appear gram-negative, while under-decolorizing makes gram-negatives appear gram-positive.

Spherical or round-shaped bacterial cells.