With the present invention, an inputted color image is separated to image signals for three colors of Red, Green and Blue, determination is made as to whether a remarked pixel is for a specified color or not according to the image signals for the separated three colors, brightness data for the inputted color image is detected, determination is made as to whether a remarked pixel is for a white color or a black color depending on the detected brightness data, a pixel determined as for a specified color is converted to a specified color, and a pixel determined as not for a specified color is converted to a white color or a black color according to a result of determination as to whether the remarked pixel is for a white color or a black color, so that a separation area for a specified color can clearly be defined.