It has been found that there are seven different classes of color matches in which the colorimetric parameters, the spectral characteristics of observer, source and specimen, are variable. The different classes result from the seven possible combinations of these three variable parameters, taken one, two, and three at a time. Those matches in which one parameter varies are called observer-variable, source-variable or specimen-variable matches; those in which two parameters vary are called source-specimen-varible, source-observer-variable, or specimen-observer-variable matches; those in which all three vary are called source-specimen-observer-variable matches. Three of these classes of color match are currently designated metameric matches, one class is currently designated spectral match, while the other three classes have no current designation. The colorimetric effect of spectral differences in all of these classes of color matches may be characterized by an index of spectral difference. Such an index has been developed by Nimeroff and Yurow for matches that are currently designated metameric matches. A generalization of this index and its applications are presented.