1. A method for controlling color reproduction performance of color reproduction devices by use of a first multi-layered feedforward neural network having an input layer including at least three units of neural circuits, at least one intermediate layer including at least one unit of neural circuit, and an output layer including at least three units of neural circuits, and a second multi-layered feedforward neural network having an input layer including at least three units of neural circuits, at least one intermediate layer including at least one unit of neural circuit, and an output layer including at least three units of neural circuits, said method comprising the steps of: training said first neural network by supplying said input layer thereof with sample color data in a first set of parameters employed by a first color reproduction device selected from CMY or CMYK in substractive color mixing and RGB in additive color mixing and by supplying said output layer thereof with sample color data according to a uniform color space, said sample color data supplied to said output layer thereof being already known to correspond to said sample color data supplied to said input layer thereof according to said first color reproduction device, while adjusting a connection coefficient of each neural circuit and biasing factors applied to each neural circuit, so that said first neural network learns a color reproduction performance of said first color reproduction device with respect to a correspondence between color data in said first set of parameters and said uniform color space; inputting to said input layer of said first neural network a set of color data of an optional color reproducible by said first color reproduction device in said first set of parameters, while outputting at said output layer of said first neural network a set of corresponding color data according to said uniform color space; training said second neural network by supplying said input layer thereof with sample color data according to said uniform color space and by supplying said output layer thereof with sample color data in a second set of parameters employed by a second color reproduction device selected from CMY or CMYK in the substractive color mixing and RGB in the additive color mixing, said sample color data supplied to said output layer thereof being already known to correspond to said sample color data supplied to said input layer thereof according to said second color reproduction device, while adjusting a connection coefficient of each neural circuit and biasing factors applied to each neural circuit, so that said second neural network learns a color reproduction performance of said second color reproduction device with respect to the correspondence between color data in said uniform color space and said second set of parameters; and inputting to said input layer of said second neural network said set of color data according to said uniform color space corresponding to said optional color reproducible by said first color reproduction device, while outputting at said output layer of said second neural network a set of corresponding color data in said second set of parameters for reproduction of said optional color by said second color reproduction device.