Abstract A series of experiments and data analyses has been performed to test the consistency between computed and perceptual color differences in images. Five International Organization for Standardization (ISO) standard color image data (SCID) images, N2, N3, N4, N5 and N7, were tested in the experiments, whose colors were altered in CIELAB lightness, chroma, and hue, either independently or simultaneously, to form the test image pairs. CIELAB, CIE94, CIEDE2000, and CMC color differences were computed by averaging the color differences pixel by pixel for the digital images or by averaging the color differences of 256 typical color patches extracted from each image for the printed images. The digital test images were displayed on an EIZO CG19 LCD monitor and the printed test images were viewed in a D50 light booth. The experimental results showed that the lightness, chroma, and hue differences behaved differently when the perceptual color differences were plotted against the computed differences. This implied that the color-difference formulas should be optimized and that different weighting factors should be applied to different visual attributes. The color-difference formulas can be optimized from the experimental data by the slope ratio of best-fit lines of lightness, chroma, and hue. The optimized formulas CIELAB(1.5:1), CIEDE2000(2.3:1), CIE94(3.0:1), and CMC(3.4:1) for digital images, or formulas CIELAB(2.4:1.5:1), CIEDE2000(2.8:1.6:1), CIE94(2.9:1.6:1), and CMC(2.7:1.5:1) for printed images, when considering hue, performed much better than the original formulas.