1. An image data processing device, comprising: a color processor that performs a color image correcting process on input color image data; a monochrome processor that performs a monochrome image converting process on the input color image data to convert the input color image data to monochrome image data, in parallel with the color image correcting process; a color/monochrome judging unit that judges, in parallel with the color image correcting process, whether an image expressed by the input color image data is a color image or a monochrome image, thereby generating a judgment result; a storage unit that stores the color image data processed by the color processor and the monochrome image data output by the monochrome processor and the judgment result of the color/monochrome judging unit; a communicating unit that sends and receives commands and drawing information to and from an external device; a transmitting unit that transmits any one of the color image data and the monochrome image data after being stored, to the external device via the communicating unit based on the judgment result of the color/monochrome judging unit stored in the storage unit; a first resolution converting unit that performs a first resolution conversion on the input color image data and provides converted color image data to the monochrome processor; and a second resolution converting unit that performs second resolution conversion on the input color image data and provides the converted color image data to the color processor.