1. A method of converting gray values of image data in a perceptual color space to K values in a CMYK printing system, the method comprising: generating a CMYK conversion model, wherein generating comprises: defining a gray region in the perceptual color space of the image data to convert the gray values of the gray region to a range of K values during CMYK color conversion of the image data; segmenting chroma values in a color lookup table, wherein the color lookup table comprises perceptual color values mapped to CMYK color values to perform the CMYK color conversion of the image data; adjusting lightness values in the color lookup table based on the segmented chroma values to substantially maintain a lightness relationship between the gray values of the image data and the remaining color values of the image data; and converting the image data to a CMYK color space using the generated CMYK conversion model.