What should I order?

Thanks for posting. It sounds like you need the S600-21 Spiral Balance. As for the question of tip color, it's generally best to stick with the original color in all cases. The tip color represents the weight range the balance is designed to support. A stronger calibration will make a lighter window very difficult to close and keep closed. The red tip of the S600-21 can support a window sash in the weight range of 6 through 19 pounds, while the blue tip can handle 10 through 29 pounds. As you can see, there's some overlap here, but if your window sash's exact weight fell outside that overlap, you'd run into issues. Therefore, best to stick with the original color unless there were issues with it to begin with. Let me know if you have further questions.