Gram staining enables observers to get a clearer view of the morphology of bacterial cells under the microscope. Bacterial cells can be rapidly stained using dyes and their cell shapes become recognizable. For instance, cocci appear spherical whereas bacilli are rod-shaped. Endospores formed by certain bacteria can also be observed via staining and microscopy. Apart from recognizing cell shapes and morphology, gram staining has also been used to classify bacteria into those that are positive to this test and those that are negative. Basically, gram-positive bacteria are those that appear violet or bluish under the microscope whereas gram-negative bacteria are those that appear pink.
Initially, all bacteria in a bacterial smear on the slide are stained violet. However, the subsequent procedures such as decolorizing and counterstaining with safranin dye will make certain bacterial cells to lose the violet stain and take the color of the counterstain (which is pink). Those bacteria are called gram-negative. In contrast, there are bacterial cells that retain the violet stain even after the whole procedure. They are referred to as gram-positive.