1. A color conversion apparatus for converting input color signals representing an input image into amounts of a plurality of chromatic colorants and at least one achromatic colorant to reproduce the input image on an image supporting member with those colorants, said apparatus comprising: means for determining an amount of the achromatic colorant from the input color signals; means for determining chromaticity values of a color of chromatic components obtained by subtracting the determined amount of the achromatic colorant from the input color signals, including: means for determining a reflectance .rho.(k') of the achromatic colorant from the amount k' of the achromatic colorant; and means for determining the chromaticity values L*, H* and C* according to equations: EQU L*=.rho.(k').sup.-1/3 (L*.sub.0 +16)-16 EQU H*=H*.sub.0 EQU C*=.rho.(k').sup.-1/3 C*.sub.0 where L*.sub.0, H*.sub.0 and C*.sub.0 denote chromaticity values of the input color signals; and means for determining amounts of the chromatic colorants for representing the color of the chromatic components from the determined chromaticity values.