Metachromasy (var. metachromasia) is a characteristic change in the color of staining carried out in biological tissues, exhibited by certain aniline dyes when they bind to particular substances present in these tissues, called chromotropes. For example, toluidine blue becomes pink when bound to cartilage. The absence of color change in staining is named orthochromasy.
Although metachromasy was observed and described since 1875, by Cornil, Ranvier and others, it was the German scientist Paul Ehrlich (1854-1915) who gave its name and studied it more extensively. The modern understanding why metachromasy occurs was advanced by Belgian histologist Lucien Lison, who studied it between 1933 and 1936 and ascertained its value in the quantitative determination of sulfate esters of high molecular weight. He also studied the metachromasy of nucleic acids.