When the idea of this game came to my mind, I had a one second thought that to make it rhythm based. However, I gave it up immediately because I had a lack of experience on both playing or making music games. Maybe one day I will have the ability to do that.
About the green zone, in the future I can switch it to yellow (yellow zone is like the default one) after the player touch it once. For now, it is OK because the mechanics and levels are not mature, so the green zone can help balance the difficulty. (I even use it frequently just to pass my own levels hahaha)
These suggestions are quite valuable. Thank you very much!!