One rule: a ball only lands on its own colour, or on empty space. That is the whole game, and it is enough.
Get it on the App StoreEvery board is generated solvable, so the moment you get stuck it was your move order, not the shuffle. That distinction matters more than it sounds: there is no point staring at a puzzle if it might be lying to you.
It gets harder by adding colours, never by taking away time. There is no countdown, nothing flashing at you, and nothing to buy that helps. Undo as far back as you like — the game would rather you found the answer than gave up on it.