Colour Sudoku: Rules and How to Play the Variant

Colour sudoku is a colour-coded logic puzzle — a sudoku variant that adds an extra constraint to the classic puzzle: as well as rows, columns, and 3×3 boxes, groups of same-coloured cells must each contain all the digits 1 to 9. It looks striking on the page — a rainbow grid — and it plays differently from standard sudoku, because the colour groups cut across the usual houses in irregular shapes. For background on the classic puzzle — invented by Howard Garns in 1979 and popularised in Japan by Nikoli in the 1980s — see Sudoku on Wikipedia.

Colour Sudoku Rules

1. Standard sudoku rules apply. Each row, each column, and each 3×3 box must contain the digits 1 to 9 exactly once.
2. The colour rule. The grid is divided into coloured cells (typically nine colours, nine cells each). Cells of the same colour must also contain all the digits from 1 to 9, with no repeats.
3. One solution. As with classic sudoku, a proper colour sudoku has a single solution reachable by logic alone.

That second rule is the entire twist — but it changes everything. In standard sudoku, each digit's placement is constrained by three houses (row, column, box). In colour sudoku, every cell belongs to a fourth house: its colour group. More constraints mean more logic to exploit — and a steeper challenge.

How the Colour Groups Work

The colour groups are irregular: unlike the neat 3×3 boxes, same-coloured cells are scattered across the grid in shapes that ignore row, column, and box boundaries. A red group might have two cells in the top-left box, three in the middle band, and four spread down the right side. This is deliberate — the irregularity is what makes the variant interesting.

Because each colour group holds exactly nine cells and must contain 1-9 once each, the colour rule behaves exactly like an extra set of nine "houses". Every solving technique you know transfers: a "hidden single" can hide in a colour group just as it hides in a row.

Colour Sudoku vs Classic Sudoku

How to Solve Colour Sudoku: Strategy Tips

• Learn the colour groups first. Before placing anything, study which cells share a colour. Trace each group with your eye until the shapes feel familiar — you will be referencing them constantly.
• Sweep colours like digits. Just as you crosshatch a digit across boxes, crosshatch a colour group: if red already contains 1-7, the two missing digits (8 and 9) can only go in red's empty cells. Check which of those cells can actually take them.
• Use the colour rule for eliminations. A cell's candidates must survive four houses, not three. When pencil-marking, remember to eliminate digits already present in the cell's colour group — beginners consistently forget this fourth check.
• Look for colour-group singles. A colour group with seven or eight cells filled gives up its missing digits quickly, and those placements cascade through the rows and boxes as usual.
• Pencil marks are essential. With four overlapping constraints, holding candidates in your head is impractical. Mark candidates from the start and update them as colour groups fill.
• Watch for colour-locked candidates. If a digit's only possible positions within a box all share one colour, that digit is locked out of that colour elsewhere — the same "locked candidates" logic as classic sudoku, extended to the new house type.

The History of Colour Sudoku

Colour sudoku belongs to the family of "extra region" sudoku variants that flourished after the puzzle's mid-2000s boom. One well-known published version is Puzzler Media's Colour Sudoku, which appeared in the Beyond Sudoku collection: its rule sheet states the variant plainly — standard rules apply, and in addition, cells of the same colour must contain all the digits 1 to 9. The concept also appears in puzzle magazines and apps under names like "color sudoku" (US spelling), all following the same extra-region principle.

Colour Sudoku for Beginners: Should You Try It?

If you are still learning standard sudoku, master the classic first — colour sudoku assumes you are comfortable with scanning, singles, and pencil marks (see our guide to how to play sudoku for beginners). Once easy 9×9 puzzles feel routine, colour sudoku is an excellent next step: it reuses every technique you know while forcing you to apply them to an unfamiliar house type, which sharpens your overall solving.

Colour-First Scanning: A Solving Method for the Variant

Standard sudoku solvers scan rows, columns, and boxes. Colour sudoku adds a fourth house — the colour group — and the solvers who thrive are the ones who promote it from afterthought to primary tool. Here is the colour-first method.

Learn your colour map before placing a digit. Spend the first minutes memorising where each colour group lives: which rows it touches, which boxes it crosses. Because the groups are irregular, this spatial knowledge is not obvious — but once internalised, it turns every colour group into a hunting ground as familiar as any row.

Scan colours for hidden singles first. A hidden single in a colour group — a digit with only one possible home among the nine scattered cells — is often invisible to row/column scanning, because the cells look unrelated. Make colour-group scanning your opening pass on every puzzle: it finds the placements that standard techniques miss, and each one constrains three ordinary houses at once.

Use colours to break deadlocks. When row, column, and box logic all stall, the colour groups are your fourth source of constraints — and because they cut across the usual houses diagonally, so to speak, they frequently unlock exactly the cells that standard scanning cannot reach. Stalled on a puzzle? Do a full colour pass before reaching for advanced techniques.

Watch for colour-box interactions. Where a colour group meets a 3×3 box, the intersection behaves like a mini-locked-set: digits confined to the intersection within the colour group cannot appear elsewhere in the box. These interactions are the variant's equivalent of pointing pairs, and they resolve some of the hardest colour-sudoku deadlocks.