Abstract: A pictorial image comprising pixels represented by a combination of density values in respect to first color components (R, G, B) is transformed to a pictorial image comprising pixels represented by combination of density values in respect to second color components (C, M, Y). In accordance with the first color modification method, an operation expressed as Q=.alpha.P+.beta. is implemented to an original density value P using two correction coefficients .alpha. and .beta. determined in advance to determine a density value Q after transformation. In addition, in accordance with the second color modification method, density values after transformation are determined by the calculation of the k-th degree masking equation using the k-th degree masking coefficients. At this time, a color cube is defined with respect to the first color components. The color cube thus defined is divided into an n number of blocks. The k-th degree masking coefficients are defined every respective blocks.