Orthochromatic

In chemistry orthochromatic refers to a dye or stain which do not change color on binding to a target as opposed to metachromatic stains which change color. In literal sense it comes from ortho ie straight or correct + chromatic ie colored. As for example Toluidine blue stains nucleic acids by its orthochromatic color (i.e., blue) but it stains mast cell granules by becoming metachromatic which in this case is red.