1. A control apparatus for controlling a printing device configured to print a color source image together with a white base by using color materials of a plurality of colors including cyan, magenta and yellow for the source image and a white color material for the white base, the control apparatus comprising: a raster-image processor that creates multi-valued pixels forming the source image and multi-valued pixels forming the white base; and a hardware processor that controls an amount of the color materials to be used in printing the source image together with the white base, by defining pixel values of the multi-valued pixels forming the source image and pixel values of the multi-valued pixels forming the white base so that a total sum of the pixel values corresponding to colors of the color materials of the source image and the pixel value corresponding to white at a same printing position becomes a color material limit or less, by using screening, converts each of the multi-valued pixels forming the source image and each of the multi-valued pixels forming the white base given after controlling the amount of the color materials, into matrixes of two-valued pixels corresponding to the colors of the color materials of the source image and a matrix of two-valued white pixels corresponding to white, to create sets of two-valued pixels corresponding to the colors of the color materials, forming the source image and a set of two-valued white pixels forming the white base, and performs a rearrangement of the two-valued white pixels, by using the matrixes of two-valued pixels corresponding to the colors of the color materials of the source image and the matrix of two-valued white pixels, given by converting each of the multi-valued pixels forming the source image and each of the multi-valued pixels forming the white base at a same printing position, the rearrangement including using the matrixes of two-valued pixels corresponding to the colors of the color materials of the source image, to determine first pixel positions at each of which at least one of the two-valued pixels corresponding to primary colors of cyan, magenta and yellow is colored and second pixel positions at each of which none of two-valued pixels corresponding to the primary colors is colored, using the matrix of two-valued white pixels, to rearrange at least one of colored two-valued white pixels at the second pixel positions in the matrix of two-valued white pixels, at a pixel position or positions at each of which a colored two-valued white pixel is absent among the first pixel positions in the matrix of two-valued white pixels, and outputting the set of two-valued white pixels forming the white base, which have been rearranged, wherein in controlling the amount of the color materials, the hardware processor uses the pixel values corresponding to the colors of the color materials of the source image, specified for each of the multi-valued pixels forming the source image, to calculate a total coverage ratio of the color materials of the source image, being a ratio of an area to be colored with at least one of the color materials of the source image to a whole area of the matrix of the two-valued pixels, for each of the multi-valued pixels forming the source image, and defines the pixel value corresponding to white, specified for each of the multi-valued pixels forming the white base, corresponding to the each of the multi-valued pixels forming the source image, such that colored two-valued white pixels exist in a coverage ratio being equal to or greater than the total coverage ratio, in the matrix of two-valued white pixels corresponding to the each of the multi-valued pixels forming the white base, and wherein in the rearrangement, the hardware processor arranges the two-valued white pixels so that the colored two-valued white pixels are located at all the first pixel positions.