A transformation from RGB model to HIS model is prosented based on the luminosity of the object and its reverse transformation. I is used to represent the brightness of the color defined as the average value of RGB, and S is used to represent the whiteness of the color defined as the minimum value of RGB divided by brightness. H is used to represent the hue of the color. Two larger values of RGB are subtracted by the minimum value of RGB. The H is defined as the ratio of the differences. In HIS model, the brightness is separated from color. Thus, the brightness has no influence on the distinction of colors. By setting certain thresholds, it is easy to distinguish red, green, cyan, magenta and yellow strips in HIS model.