MGG stain is used to stain blood, bone marrow, and cytological specimens. It contains basic dyes that stain nuclei and granules blue and an acidic dye that stains red blood cells and eosinophil granules red. The basic dyes stain cellular components due to their positive charge binding to negatively charged DNA and RNA, while the acidic dye stains due to its negative charge. Proper pH is critical for the staining reaction to work correctly.