to_cmyk (Color::RGB)

    # File lib/color/rgb.rb, line 168
      def to_cmyk
        c = 1.0 - @r.to_f
        m = 1.0 - @g.to_f
        y = 1.0 - @b.to_f

        k = [c, m, y].min
        k = k - (k * brightness)

        c = [1.0, [0.0, c - k].max].min
        m = [1.0, [0.0, m - k].max].min
        y = [1.0, [0.0, y - k].max].min
        k = [1.0, [0.0, k].max].min

        Color::CMYK.from_fraction(c, m, y, k)
      end