A method of color correction has been developed based upon the observation that any color within the gamut of the four-color process printing inks can be reproduced by pairs of the chromatic inks plus black. The chromaticity coordinates of a color will, vary with the two chromatic ink mixtures, whereas the luminance is controlled by the amount of black used. A device is developed in which the empirical relationship existing between the chromaticity of a given copy and the amounts of each of the chromatic inks required for match is readily employed. The luminance of the copy is then matched by addition of black. Inherently, this process has the advantage of basic simplicity, allowing empirical compromises or corrections for distortion.
PHILIP E. TOBIAS, "Color Correction Process," J. Opt. Soc. Am. 45, 535-535 (1955)