Thank you for playing and all the suggestions.
I realize that getting used to the color mapping can be a bit frustrating, but I couldn't think of a good way to indicate their configuration without partially obstructing the game. I have added the ability to display hints that the player can toggle in when choosing the song. (see screenshot)
I have moved loading the song to a separate thread, so now there is an indication that the game isn't hanging.
The problem with allowing window resizes is that it would alter the mechanics of the game - if the screen were to be widened, the player would have more time to shoot the bullets coming from the sides. I could add an option to resize the screen, but keep the ratio.
I think the game should be ready for an open beta soon, but it would be good to hear if it works for others too.