No scheme for calculating a color obtained by mixing two spot colors (composite spot color) has been established yet. Hence, an image processing apparatus of this invention includes a spectral database which stores spectral data of spot colors and mixed colors of the spot colors. Upon rendering, a mixed color region of spot colors is detected. A colorimetry table of the spectral DB which corresponds to a combination of the spot colors of the mixed color region is selected with reference to a combination table of the spectral DB. Spectral data corresponding to a mixed color is obtained from the colorimetry table on the basis of the density values of the spot colors of the mixed color region, and the obtained spectral data is supplied to rendering processing.