In chemistry, a color reaction or colour reaction is a chemical reaction that is used to transform colorless chemical compounds into colored derivatives which can be detected visually or with the aid of a colorimeter.
The concentration of a colorless solution cannot normally be determined with a colorimeter.  The addition of a color reagent leads to a color reaction and the absorbance of the colored product can then be measured with a colorimeter.
A change in absorbance in the ultraviolet range cannot be detected by eye but can be measured by a suitably equipped colorimeter.  A special colorimeter is required because standard colorimeters cannot operate below a wavelength of 400 nanometers.  It is also necessary to use fused quartz cuvettes because glass is opaque to ultraviolet.