def to_gui
      return to_standard_hex if hexadecimal?
      return Convert.xterm_to_hex(@value) if cterm?
      return :NONE if none?

      raise ColorError.new "can't convert \"#{ @value }\" to gui"
    end
def to_gui
      return to_standard_hex if hexadecimal?
      return Convert.xterm_to_hex(@value) if cterm?
      return :NONE if none?

      raise ColorError.new "can't convert \"#{ @value }\" to gui"
    end
Convert the color value to a hexadecimal color

 @return [Symbol,String]
   the color as either "NONE" or hexadecimal

 @api public