Metachromasia (also known as metachromacy) is a characteristic color change which certain aniline dyes exhibit when bound to particular substances or when concentrated in solution. For example, the basic dye toluidine blue becomes distinctly pink when bound to cartilage matrix. In the sense used here, the metachromasia refers to a change in color not observed with normal tissues, anomalous staining with the cationic dyes toluidine blue O and Alcian blue resulting from excessive amounts of the polyanionic glycosaminoglycans. [ PMID:4195824 HPO:probinson ]