This has the basis of a fantastic mobile game. The mechanic is very simple and straight-forward, and I could just picture myself playing this on the way to work. There was a lot of polish too like the intro animation and transitions.
Now, for improvements, I agree with n.s.chaitanya on adding another side or two. I think that really should have been what the difficulty selector changes instead of the timer. Or, if you really want to keep the timer-related difficulty, just scale it up over time so you have less and less time to pick the correct side each time.