Metachromasia is a histological term used to describe a change in the (dye-typical) coloration of certain biological structures. In other words, certain tissue structures stain with a different color than the original dye.
This method is used to visualize mast cells, amyloid deposits, fibrioid degeneration and other changes. Dyes with metachromatic properties are e.g. Congo red, toluidine blue, methyl violet.