Heyo~
I thought the visuals were very pleasant and the execution pretty solid.
For my suggestions, I think that this game would benefit a lot from a sound effect when the user successfully hits a key. Also, when transitioning from which you need to hit the exact timing of it was kinda unclear to me. Maybe a visual representation of what you should currently be targeting would be really helpful.
I had a lot of fun playing this one! Well done :)