1. A color reduction processing apparatus, comprising: an image data acquisition unit configured to acquire RGB color image data defined by RGB color values; a conversion processing unit configured to convert the RGB color image data to HLS color image data defined by HLS color values; and a color reduction processing unit configured to convert the HLS color image data to print image data that comprises a first color, a second color, and a non-printing color based on specific parameters; a parameter setting unit configured to set a hue range Hs for the second color, wherein the color reduction processing unit converts the HLS color image data to print image data based on the hue range Hs setting; and wherein the first color is black, and the color reduction processing unit comprises: a hue ratio calculation unit configured to calculate a hue ratio Hr based on the equation: 
 Hr= 1−| |180 −|Hp−H 2| |−180|/ Hs, where Hp denotes the hue of a target pixel and H 2 denotes the hue of the second color, where, if Hr≦0, then Hr is set to 0; a lightness ratio calculation unit configured to calculate a lightness ratio Lr between the lightness of the target pixel Lp and the lightness of the second color L 2 based on the equation: 
 Lr =(1 −Lp )/(1 −L 2), where if Lr≧1, then Lr is set to Lp/L 2 , and Lp and L 2 are set to values in the range of 0.00 to 1.00; a saturation ratio calculation unit configured to calculate a saturation ratio Sr between the saturation of the target pixel Sp and the saturation of the second color S 2 based on the equation: 
 Sr=Sp/S 2, where if Sr≧1, then Sr is set to 1; a second color data calculation unit configured to calculate a second color ratio R 2 based on the hue ratio Hr, the lightness ratio Lr, and the saturation ratio Sr based on the equation: 
 R 2 =Hr*Lr*Sr, where * denotes multiplication; and a first color data calculation unit configured to calculate a first color ratio Rk based on the equation: 
 Rk= 1 −Lp−R 2*(1 −L 2).