Pour coloured liquid between tubes until each one holds a single colour. That is the whole game, and Vial tries to be the version of it that respects you.

Every level is checked before it ships. A solver proves each one solvable from its starting position, so you are never handed a board that was dead before you touched it. That sounds obvious. Most of this genre does not do it.

Difficulty is measured, not guessed. Every level is played thousands of times by a simulated player before it reaches you, and its difficulty is that player's failure rate. The levels the campaign uses are then measured again, far more carefully, because ordering two levels whose difficulty differs by less than the measurement's own error is not ordering them at all. Chapters introduce one new idea at a time, and the campaign is a hundred and eighty levels drawn from a pool of five thousand.

Nothing is rationed. Undo as much as you like. Hints are free and come in three steps, so you can take the smallest amount of help that unsticks you — ask only whether the board is still winnable, ask for a tube worth looking at, or ask for the exact move. There are no adverts, no currency, no lives, no energy timer and nothing to buy. There is no version of this app that sells you a way past its own friction.

Three stars, and you can see what they cost. Every level carries a par: the shortest solution that exists, worked out before the level shipped. Finish at par, unaided, without leaning on undo, and you keep all three.

Some levels hide what you have not uncovered. A veiled tube shows only down to its first colour change, and once you have seen a colour it stays seen. Uncovering a colour uncovers all of it at once, so you are never made to pour one unit at a time to satisfy a rule. What is hidden is knowledge, and knowledge is the puzzle.

Built to be playable if you cannot separate colours. The palette was chosen by searching for the arrangement with the best worst-case separation under protanopia, deuteranopia and tritanopia — not picked by eye. An optional symbol on every unit is one tap away in settings, and a reduce-motion setting stills the board.

Completely offline. No account, no sign-in, nothing collected, and no server anywhere — there is nowhere for anything about you to go. The daily puzzle works on a plane. Your progress lives on your phone and you can move it to a new one yourself.

Sound you can switch off. Pours, completed tubes, wins, and a quiet loop underneath. Effects and music have separate switches, and both of them do something.

Campaign, endless mode that matches how you are actually playing, and a daily puzzle that is the same for everybody.

Thomas van Tilburg

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