1. A method for describing an image region based on color histogram, comprising: transforming the image region into HSV or HSL color space; extracting a specific color range, which includes colors perceptible by human vision as black, gray and white colors, from the HSV or HSL color space; dividing the HSV or HSL color space except for the specific color range into a plurality of common color ranges; quantizing the image region into a quantized color region by quantizing the specific color range according to a Value plane of the HSV color space or a Lightness plane of the HSL color space and quantizing each of the common color ranges according to the Value plane and a Saturation plane of the HSV color space or the Lightness plane and a Saturation plane of the HSL color space; computing one or more color histograms according to the quantized color region; smoothing histogram bins corresponding to the common color ranges except for the specific color range in the one or more color histograms so that adjacent histogram bins have an impact on each other; and generating a unique vector by using the one or more color histograms.