1. An image processing device comprising: a memory; a rasterizer configured to generate raster data expressed in a first color space by rendering an object in the memory, the object corresponding to a drawing command; a color space converter configured to convert the raster data from the first color space to a second color space capable of being set so as to reproduce gray of gray pixels in the object by any one of a black coloring material and mixture of multiple color materials, wherein: the rasterizer determines whether the gray of the gray pixels is to be reproduced by the black coloring material or the mixture of multiple color materials based on an attribute of the object and adds gray reproduction information on how to reproduce the gray for each gray pixel to the raster data, the gray reproduction information being obtained from the determination; and the color space converter sets pixel levels in the second color space based on the gray reproduction information.