PURPOSE: To accurately reproduce a color under an arbitrary light source by using the color value obtained from the spectral distribution of illumination for observation and the color value corresponding to a prescribed light source to convert the color to the device value of a color output device.

CONSTITUTION: A spectro photometric-colorimetric instrument 7 sends a spectral distribution signal 8 of the illuminating light source for observation to a color value calculating computer 1. The computer 1 calculates a color value signal 2 corresponding to the illuminating light source for observation and sends it to a color conversion means 4. The computer 1 sends a color value signal 3, which is preliminarily registered and corresponds to a prescribed light source like a D light source, to the color conversion means 4. This means 4 consists of a feed forward neutral network with the learning function and used color value signals 2 and 3 to correct a device value signal 6 of RGB or the like and sends it to a color output device 5. Thus, the color output device 5 reproduces the picture whose color under the illuminating light source for observation looks to be equal to that under the prescribed light source.