Auxochrome

An auxochrome (Greek αὐξάνειν auxánein: "to increase" and χρῶμα chrōma: "colour") is a group of atoms attached to a chromophore which modifies the ability of that chromophore to absorb light. They themselves fail to produce the colour; but when present along with the chromophores in an organic compound intensifies the colour of the chromogen. Examples include the hydroxyl group (-OH), the amino group (-NH₂), the aldehyde group (-CHO), and the methyl mercaptan group (-SMe).