1. A method of sharing a color reproduction by a plurality of color image output devices for input device-dependent image data, for sharing, as a second profile for a second output device, a known first profile for converting the input device-dependent image data into first output device-dependent image data depending on a first output device by performing a pre-gradation converting process, a color converting process, and a post-gradation converting process on the input device-dependent image data, the method comprising the steps of: determining a differential between a gradation conversion curve of the first output device and an inverse conversion curve of a gradation conversion curve of the second output device; and using the first profile and the differential as the second profile for the second output device.