def highlight(mark = PASS)
      case mark
      when PASS; rainbowify PASS_ARY[@color_index%2]
      when FAIL; "\e[31m#{mark}\e[0m"
      when ERROR; "\e[33m#{mark}\e[0m"
      when PENDING; "\e[33m#{mark}\e[0m"
      else mark
      end
    end
def highlight(mark = PASS)
      case mark
      when PASS; rainbowify PASS_ARY[@color_index%2]
      when FAIL; "\e[31m#{mark}\e[0m"
      when ERROR; "\e[33m#{mark}\e[0m"
      when PENDING; "\e[33m#{mark}\e[0m"
      else mark
      end
    end
Determines how to color the example.  If pass, it is rainbowified, otherwise
 we assign red if failed or yellow if an error occurred.

 @return [String]