Final answer:
Methylene blue is a basic stain used for staining negatively charged cell walls, while methyl red (sometimes confused with methylene red) is an acidic dye used as a pH indicator.
Step-by-step explanation:
Methylene blue is a type of basic stain, which is used in microbiology to stain cells that typically have negatively charged cell walls. Since the positive chromophores in basic dyes like methylene blue are attracted to the cell walls, they effectively stain these structures. Methylene blue functions as a positive stain because it colorizes cells making their structures easily visible under a microscope. Conversely, there is no commonly used dye known as methylene red; however, methyl red is an acidic dye that functions as a pH indicator in microbiology for detecting acid production by bacteria during fermentation processes.