A description of the D & H Color Rule and its use as a color vision test for both normal and abnormal color vision, and also its use to judge changes in the quality of illumination from different light sources. The Rule is made up of a series of strongly metameric colors, a pair of which will appear to match to one observer under a particular source of illumination, but will not necessarily match to another observer. It can also be used by one observer to compare different light sources. -- AATA