The staining of tissue with a dye in a predictable way; a green dye stains the tissue green. What is this called?

• A. Orthochromasia
• B. Metachromasia
• C. Counterstain
• D. Differentiation

Answer: (A)

The main idea here is whether a dye makes the tissue appear in the dye’s own color or whether the tissue causes the dye to change color. Orthochromasia describes staining where the tissue takes on the true color of the dye—so a green dye staining tissue green is orthochromasia. Metachromasia would be when the tissue causes the dye to shift to a different color, such as a blue dye appearing purple or red in certain tissue components. Counterstains are used to add contrast after the primary stain, not to describe the primary dye’s color outcome. Differentiation refers to the process of removing excess stain to distinguish structures, not to the color the tissue adopts.