Metamerism, with reference to an arbitrary pattern and any ternary combination of colorants suitable for matching its color, is generally not antisymmetrical with respect to any pair of illuminants. For the exceptional case of antisymmetry, the prerequisite, necessary, and sufficient condition is formulated. There are, therefore, two measures of metamerism, and not one. These two measures are vectors that differ both in sign and generally in absolute value of all three components. <i>A priori</i>, there is no reason to prefer any one of the two measures of metamerism—but there may be technical reasons for doing so. Formulas are deduced for the practical computation of both of these vectors. These are applicable even when the colors of the samples differ in both of two illuminants.
Ulrich J. Gugerli, "Lack of antisymmetry in metamerism," J. Opt. Soc. Am. 63, 755-760 (1973)