As a musician I love game that's utilizing music as game mechanic.
BUT phrygian, locrian etc are modes. Even I don't know them by the name (except phrygian because that's most used mode for metal), so I think that's kinda to high level for non musicians.
Color choice is bad for colorblind people, I'have struggle with green color myself and it seems black key highlight was my weak spot. For those cases try to give secondary way to give separated look, for example dotted line.
I'd say you have a bug on detecting the rule of one step apart - it doesn't wrap from other end. So if player will get C Phrygian it's dead end (C is most left key and closest key to the right is half-step, but A# from other side doesn't count).