A first set of different color signals represented in a colorimetric system is converted into a second set of different color signals represented in another colorimetric system. Predetermined main parts of the second set of the color signals are determined in response to predetermined first portions of the first set of the color signals. Calculation coefficients are determined in response to the first portions of the first set of the color signals. Predetermined subordinate parts of the second set of the color signals are calculated from predetermined second portions of the first set of the color signals and the calculation coefficients according to an interpolation. The determined main parts of the second set of the color signals and the calculated subordinate parts of the second set of the color signals are combined into a whole of the second set of the color signals. The calculation coefficients are designed so that, if the color signals in the first set vary by equal values, achromatic color data which results from the calculation lies on an achromatic color axis in a color space defined with respect to the readout data of the main parts of the second set of the color signals, and that the sum of squared errors of color conversion is minimized.