def next(n=1)
      @@colorset[mode] ||= Colorable::Colorset.new(order: mode)
      idx = @@colorset[mode].find_index(self)
      @@colorset[mode].at(idx+n).tap{|c| c.mode = mode } if idx
    end
def next(n=1)
      @@colorset[mode] ||= Colorable::Colorset.new(order: mode)
      idx = @@colorset[mode].find_index(self)
      @@colorset[mode].at(idx+n).tap{|c| c.mode = mode } if idx
    end
Returns a next color object in X11 colors.
 The color sequence is determined by its color mode.