What is "orthochromasia"

In chemistry, orthochromasia is the property of a dye or stain to not change color on binding to a target, as opposed to metachromatic stains which change color. The word is derived from the Greek orthos (correct, upright), and chromatic (color). Toluidine blue is an example of a partially orthochromatic dye, as it stains nucleic acids by its orthochromatic color (blue), but stains mast cell granules in its metachromatic color (red).

In spectral terms, orthochromasia refers to maintaining the position of spectral peaks, while metachromasia refers to a shift in wavelength, becoming either shorter or longer.

In photography, an orthochromatic light spectrum is one devoid of red light.

In biology, orthochromatic refers to the greyish staining because of acidophilic and basophilic mixture in the cell.