A small suggestion: when you press the purple buttons, it would be neat to see the green numbers change by the multiplier (or have an option to do so). This would let you easily see parts of the keyboard transpose correctly vs not, and what new pitches you get access to. Even sweeter would be a quick animation to show "where the numbers went".
I would also like to be able to have the purple buttons be active only when pressing the keys, so that I could quickly modulate up a just fourth and back. Having to remember where I am in the modulation state is a bit much, and very different from typical musical instruments.
But I've been playing with this for a good while, trying to recreate basic stuff like V7 -> I etc and it's very cool, thanks again.