1. An image data processing apparatus for processing multi-color image data including first image data of a first color and first image data of a plurality of primary colors, received from a host device to embed an electronic watermark, the image data processing apparatus comprising: an image generator for generating, from the first image data of the first color and the first image data of the primary colors, a second image data of the first color and second image data of the primary colors; an image converter for converting the second image data of the primary colors to generate first image data of a second color; a processing unit for processing the first image data of the second color and the second image data of the first color to generate a second image data of the second color; a decision unit for dividing the multi-color image data into a plurality of blocks representing different image areas and deciding, based on the second image data of the second color in said each block in the plurality of blocks, whether the block is watermarkable; and a watermarking unit for embedding the electronic watermark into each watermarkable block by modifying at least the first image data of the first color.