I really like this idea! Maybe the 4th color will be pale green (aka you're close)
> Another recommendation: make an easy mode in a single octave for practice. Allow progression to more octaves once easy mode is mastered.
This is unfortunately hard to do in the regular daily game cadence because I'd have to limit myself to songs that span only one octave within the first 6 notes, but I can definitely see it as an easy mode feature in "practice" mode. I'm also thinking that ironically, fixing the octave makes it easier for users because it limits the choices, so maybe:
1. easy: the octaves switch for you
2. normal, hard: you have free reign to switch octaves, but right_note && wrong_octave === pale green tile
Recommendation: add a 4th color for correct note, wrong octave.
Err on the side of just making it wrong.
Another recommendation: make an easy mode in a single octave for practice. Allow progression to more octaves once easy mode is mastered.