1. A method of converting gray values of image data in a CIEL*a*b* 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-only values of an ICC color profile during CMYK color conversion of the image data; segmenting chroma values in a color lookup table, wherein the color lookup table comprises CIEL*a*b* 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, wherein segmenting chroma values in the color lookup table comprises: clipping a* and b* color values on input curves of the perceptual color space to 0.5; and smoothing ends of the defined gray region to monotonically increase the input curves, and wherein adjusting the lightness values in the color lookup table comprises: comparing L* values with neighboring L* values in the color lookup table; and using a Heimholtz-Kohlrausch Effect perceptual model to adjust corresponding K values in the color lookup table to adjust the lightness values of the converted image and clarify abrupt transitions of the gray region in the converted image.