Gram Positive Vs. Gram Negative 

Many years ago, a man named Christian Gram discovered a method for classifying bacteria using a colored stain. To do this,  a lab worker drops a bit of violet color on a plate that has been swiped with bacteria. Then, the stain is rinsed off using different chemicals. If the bacteria on the plate have a thin outer layer, the stain will be absorbed and remain in the bacteria. When using a microscope, the bacteria will appear purple or blue tinted. We call these bacteria gram positive. 

Gram-negative bacteria have a thick outer layer made up of lipids/fats. When the violet stain is rinsed away, it’s largely removed because it was not absorbed by the bacteria. These bacteria will have a red color when magnified.