PURPOSE: To set lattice points outside of a color reproducing area and picture signals outside of the color reproducing area without generating a rapid change in colors within the color reproducing area by repeating the addition of the lattice points within the color reproducing area and maintaining an initially set value for the lattice point outside of the color reproducing area.

CONSTITUTION: A lattice for which the entire L*a*b* color space is turned to one area is set. Then, a reference color picture recording signal YMC is set and color patches are outputted from it by a printer. The color patches are color-measured and the L*a*b* values of the respective patches to be utilized as sample data are obtained. A reference color interpolation color picture recording signal YMC is obtained by interpolation using the lattice of the L*a*b* space, the value is compared with the reference color picture recording signal YMC of the sample data, the values of the lattice points of the L*a*b* space are adjusted so as to minimize the mean square of the distance of the two picture recording signals and the lattice is divided. The adjusted and divided lattice is utilized for the interpolation again.