This is a method (named after the Danish bacteriologist, Hans Christian Gram) for staining and identifying bacteria.
About Gram Stains
The Gram stain test, developed in the 1800s by Hans Christian Gram, is a method for classifying different types of bacteria using a chemical stain and viewing through a microscope the results on the bacteria‘s protective cell wall.
Most bacteria are classified into two groups—Gram-positive or Gram-negative—depending on whether they retain a specific stain color.
Gram-positive bacteria retain a purple-colored stain, while Gram-negative bacteria appear pinkish or red. NIH – National Institute of Allergy and Infectious Diseases