# File lib/highline/string_extensions.rb, line 106
        def on_rgb(*colors)
          color_code = colors.map{|color| color.is_a?(Numeric) ? '%02x'%color : color.to_s}.join
          raise "Bad RGB color #{colors.inspect}" unless color_code =~ /^[a-fA-F0-9]{6}/
          color("on_rgb_#{color_code}".to_sym)
        end