• Every square holds one of the two colors.
• No three of the same color next to each other in a row or a column.
• Every row and every column holds the same number of each color.
• No two rows may match, and no two columns may match.

Stuck? When looking at squares two or three at a time stops giving you anything, take one whole row or column and list every filling that still fits it. Whatever all of those fillings agree on is certain. Often only one of them avoids three of a color in a row, and that single line is enough to restart the rest of the grid.

Tap a square to cycle it through the first color, the second color, and empty again. Squares that came with the puzzle are fixed. With a keyboard, the arrow keys move between squares and Enter or Space fills one.