Abstract: The color gamut of different color reproduction devices differs in size and shape,so the compression of the color gamut must be carried out when a RGB image file is transformed into a CMYK image.There are rich gray levels in the out-of-gamut of an image.But the existing compression methods do not pay enough attention to it.This paper proposes a new compression method,named"Tagging method".Using this compression method,we can not only fulfill the compression from RGB color gamut to CMYK color gamut and keep the most colors in the gamut invariable,but ensure the number of gray levels in the compressed image area.This paper also proposes a detected method for judging the out-of-gamut colors.The experiment shows that out-of-gamut of an image processed by this detected method is very similar to that processed by"Gamut Warning" in Photoshop.