a color image forming apparatus provided with an input terminal for plural input color component signals , an independent input terminal for an output color component control signal , a processing circuit for processing the input color component signal to obtain plural output color component signals specific to the apparatus , and image forming means for forming each color component image according to the value of each output color component signal , in which the processing circuit is capable of controlling the calculation of output color component control signals according to the value of the output color component control signal . in one illustrated embodiment , the output color component signal is adapted to maintain a black output color component at a predetermined value .