I am an older gentleman, at 66 yo I like the app. Trying to learn notes and this app is very helpful, but a couple of things that may make it better. First off the size and color of the selectable notes could be bigger, not huge just a little bigger, the colors could be a little more opaque the actual letter is hard to see.
Also a little side note to your users would be to write down on lined
paper the notes in each position it helps in remembering them.
Otherwise nice job. Steve Plante