Metachromasia

Metachromasia is a characteristical change in the color of staining carried out in biological tissues, exhibited by certain dyes when they bind to particular substances present in these tissues, called chromotropes. For example, toluidine blue becomes dark blue when bound to cartilage. Other widely used metachromatic stains is the family of Romanowsky stains that also contain thiazine dyes: the white cell nucleus stains purple, basophil granules intense magenta, whilst the cytoplasms stains blue, which is called the Romanowsky effect. The absence of color change in staining is named orthochromasia.

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