1. An image data processing device capable of outputting image data to one or more image display devices comprising: a first image acquisition unit for obtaining first image data; a first image output unit for outputting the first image data to the image display devices; an attention area acquisition unit for obtaining an attention area composed of one or more arbitrarily specified pixels from the first image outputted to the image display devices; a color chart display unit for displaying a color chart whose hue and chromes are gradually changed toward around the color corresponding to the image data of the attention area; a selected color information acquisition unit for obtaining arbitrarily selected color information from the displayed color chart; a color parameter calculation unit for obtaining a plurality of combinations of the attention area and the color information corresponding to the attention area and calculating a color parameter on the basis of the obtained plurality of combinations; a color conversion unit for converting the color of the obtained image data on the basis of the color parameter; and a second image output unit for outputting the image data whose color is converted, to the image display devices as a second image.