Input color signals are input at step S11 and are converted into first CMYK color material amounts to minimize a first distance function at step S12. At step S13, color coordinates reproduced from the first CMYK color material amounts are obtained based on a printer model. If the input color signals are determined within a color reproduction range at step S14, the first CMYK color material amounts are output at step S22. If the input color signals are determined outside the color reproduction range, reproduced lightness is compared with input lightness at step S15. If the reproduced lightness is less than the input lightness, lightness weight .alpha. responsive to a yellow saturated color rate is used to generate a second distance function at steps S16-S18. On the other hand, if the reproduced lightness is equal to or greater than the input lightness at step S15, a second distance function having weight as a constant is generated at steps S19 and S20. Second YMCK color material amounts to minimize the second distance function are calculated at step S21 and are output at step S22.