Using a large number of specially selected imaginary object colors which are metameric with respect to one set of color-mixture functions, the spatial distribution of these colors with respect to the other set of color-mixture functions provides an illustrative means of measuring the total difference of the two sets of color-mixture functions. The spatial distribution follows a normal trivariate distribution law which allows the computation of an ellipsoid that is expected to contain 95% of all theoretically and practically possible object colors of the same class used to calculate that ellipsoid. A numerical example involving the color-mixture functions of the 1931 CIE standard observer and the color-mixture functions derived from the Stiles 10° pilot data demonstrates the theory.
GÜNTER WYSZECKI, "A Measure for the Total Difference of Two Sets of Color-Mixture Functions," J. Opt. Soc. Am. 49, 811-813 (1959)