Eosin Y is a slightly acid fluorescent dye, which stains cytoplasm red. The dye is used for the differential staining of connective tissue and cytoplasm, spore staining and in histopathology as a counterstain after hematoxylin and before methylene blue. Tested for use as histological stain and as fluorescent indicator ( Y = yellowish). Eosin Y is a slightly acid fluorescent dye, which stains cytoplasm red. The dye is used for the differential staining of connective tissue and cytoplasm, spore staining and in histopathology as a counterstain after hematoxylin and before methylene blue. Tested for use as histological stain and as fluorescent indicator ( Y = yellowish).