1. An image data generating apparatus comprising: at least one processor configured to execute: an extracting unit configured to extract one or more pixels corresponding to a metallic-color region expressed by a metallic color, by using lightness and saturation of a read image and lightness and saturation of a captured image, the read image being obtained by reading a document having the metallic-color region, the captured image being obtained by capturing an image of the document; and a generating unit configured to generate image data in which the one or more pixels corresponding to the metallic-color region and extracted by the extracting unit are colored in the metallic color.