A hematoxylin-metal complex acts a as a basic dye, staining nucleic acids in the nucleus and the cytoplasm blue, brown, or black. Eosin is an acid aniline dye which stains the more basic proteins within cells (cytoplasm) and in extracellular spaces (collagen) pink to red. Cartilage and mucus may stain light blue. Hematoxylin, a basic dye, binds to acidic components of a tissue, which are thus said to be "basophilic." The color of the stained structures depends on the mordant used to make the hematoxylin dye bind to the molecules of the tissue. Potassium alum, the most common mordant, gives the dye a blue to purple color. Eosin, an acidic dye, binds to basic components of a tissue, which are thus said to be "acidophilic." The structures stained by eosin are typically colored pink to red.