There is described in the present paper a method, discovered empirically, for computing the approximate number of “least perceptible differences” between any two colors of the same brightness whose specifications are available. This method has been stated in simple form as an empirical relation; it is shown to be in substantial agreement with extant sensibility data of the following types: (1) the least wave-length difference perceptible in the pure spectrum as a function of wave length (Steindler, Jones); (2) the least dominant-wave-length difference perceptible at constant purity as a function of purity (Watson, Tyndall); (3) the least purity difference perceptible at constant dominant wave length as a function of purity (Donath); (4) the least purity difference perceptible near zero purity as a function of dominant wave length (Priest, Brickwedde), and (5) the least color-temperature difference perceptible as a function of color temperature (Priest). A mixture diagram is included showing colors specified by their trilinear coordinates and by the dominant wave length and purity of their stimuli. From this diagram the number of “least perceptible differences” separating any two colors of the same brightness may be read with a degree of certainty indicated by the comparisons here presented.