This paper proposes a method of six-color separation using additional colorants and a quantitative granularity metric to reduce the color difference and graininess. In conventional six-color separation, the use of light magenta and light cyan is generally an effective way of reducing the visibility of dots in bilevel printing devices, i.e., light magenta and light cyan are used in a bright region instead of magenta and cyan. However, the hue values for light magenta and light cyan differ from those for magenta and cyan in CIELAB space, making the colorimetric reproduction less accurate. Therefore, to minimize this inaccuracy, the proposed method uses yellow and light magenta colorants as additional colorants. In a bright region, magenta is replaced with light magenta and yellow, while cyan is replaced with light cyan and light magenta. These combinations reduce the hue difference, as they create colors with a similar hue to magenta and cyan. In addition, a smooth image can simultaneously be obtained owing to the lower dot visibility of the additional colorants. In a middle region, magenta is replaced with light magenta and magenta, while cyan is replaced with light cyan and cyan. Since the use of these two colorants with different concentrations makes a coarse dot pattern, this phenomenon is reflected based on a quantitative granularity metric. Finally, in a dark region, only the magenta and cyan colorants are used as usual. Experiments show that the proposed method can simultaneously produce a colorimetric and smooth tone reproduction.