A color-tone similarity index (CSIM) between two color images is presented. CSIM is defined by a statistical analysis of cumulative histograms in a hue-oriented color space. It characterizes the color distributions, while the existing structural similarity index reflects the spatial structure involved with grayscale images. The behaviors of CSIM are checked by the comparisons of color code chips. Through an image quality assessment on TID2008, the correlation between CSIM and the mean opinion score was proved to be statistically significant.