You’re given a grid, with several dotted pieces on lighter squares. Your objective is to fill every empty space with one of two pieces, a purple square or a yellow disc, while following these rules:
On a touch screen, a tap does what a click does, and a long press what a right-click does (more ways to right-click).
Click a square to color it purple (click again for yellow, and again to clear it). Right-click it to color it yellow (again for purple, and again to clear it).
With the keyboard, the arrow keys move a cursor around the board. Enter does what a click does to the square under it, and Space what a right-click does. Press 1 to color it purple; clicking it round gets there too. Press 0 or 2 to color it yellow; right-clicking it round gets there too. Press Backspace or Delete to clear it; clicking it round gets there too.
Drag from a square to paint every square you pass over the color the press gave the first one. Hold Shift while moving the cursor to paint the squares it passes over yellow, Ctrl to paint them purple, or both to clear them.
This puzzle type was invented by Inaba Naoki under the name クラスター, or literally Kurasuta, which Google Translate names ‘Cluster’. The original puzzles are available here: http://www.inabapuzzle.com/honkaku/kura.html
Hint explains the next step rather than simply making it. Each step decides the color of one square, by showing that the other color would break a rule. It reasons from the dots and the squares you have colored, so it carries on from wherever you are.
On Normal boards a step reads two tiles’ counts together, over the squares both touch. A dot that needs one more of its color, where the other tile’s count leaves at least one of the shared squares that color, has its last one there, so its other empty neighbors take the other color. A tile that needs more of its color than the other tile’s count allows among the shared squares takes its color on its other neighbors. The hint outlines both tiles.
On Unreasonable boards there may come a point where no deduction is left and the only way on is to try a color and see whether it works. The hint says so rather than guessing for you: save your position, try it, and undo if it breaks.
The rules a step shows breaking are the ones the puzzle states: a dot touches exactly one square of its own color, and every other square touches at least two. A square whose neighbors are all the other color, or that can only ever match one of them, can’t be that color.
If a square you colored is the wrong color, the hint asks you to fix the highlighted mistakes first, and the square’s background turns red, as it does for Check & save, whether or not it breaks a rule yet. The red frame is a different mark: it is drawn as you play, round a square or dot whose rule is broken, and that one may itself be right.
These parameters are available from the ‘Custom type…’ option on the ‘Type’ menu.
Easy puzzles can be finished a square at a time: for each empty square, one of the two colors breaks a rule outright, so the other one is forced.
Normal puzzles also need two tiles' counts read together over the squares both touch. A board an Easy player could finish is not offered as Normal.
Unreasonable puzzles may need you to try a color and follow where it leads.
Normal needs a board with room for the deeper reasoning, so it is not offered on very small grids.