The Free Color Sudoku with a 4th Constraint
Rainbow Technique
Because every digit spans all 9 colors, tracking it across the grid reveals the missing one. The digit reveals the color.
A color sudoku technique that has no equivalent in classic sudoku.
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Chromatic Circle
The 4th rule works symmetrically: each color spans all 9 digits, so tracking it exposes the missing one. The color reveals the digit.
An advanced free online sudoku strategy exclusive to this game.
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One extra rule rewrites the game.
Every color carries all 9 digits.
What is Suirodoku?
Suirodoku — also known as Tsuidoku — is a free online color sudoku built on a revolutionary 4th constraint: every color must also contain all 9 digits. Combined with the classic row, column and block rules, this generates 81 unique digit-color combinations and unlocks solving strategies impossible in classic sudoku.
The 4th Rule That Changes Everything
Classic sudoku has 3 constraints. This free online color sudoku adds a 4th — and it changes everything:
- Each row contains numbers 1-9 and all 9 colors
- Each column contains numbers 1-9 and all 9 colors
- Each 3×3 region contains numbers 1-9 and all 9 colors
- Each color contains all 9 digits — the 4th constraint that unlocks new solving techniques
81 Unique Pairs
The 4th color constraint produces a perfect bijection: each of the 81 digit-color combinations is unique across the entire grid. While classic sudoku repeats each digit 9 times, every cell here holds an irreplaceable identity — making this the most demanding free online sudoku available.
81 cells.
81 combinations.
0 duplicates.
1 solution.Sudoku vs Suirodoku
Sudoku
- 3 constraints
- Numbers only
- Each number repeated 9 times
- Classic techniques
Suirodoku
- 4 constraints: rows, columns, regions + colors
- Colors are full players, not decoration
- 81 unique digit-color pairs, 0 repetition
- Rainbow & Chromatic Circle exclusive techniques
Frequently Asked Questions
What makes Suirodoku different from Sudoku?
Suirodoku adds a 4th constraint to classic Sudoku: every color must also contain all 9 digits. This single extra rule creates 81 unique digit-color pairs and unlocks two exclusive solving techniques — Rainbow and Chromatic Circle — that are impossible in classic Sudoku.
What is the 4th constraint in Suirodoku?
Like Sudoku, each row/column/3×3 region must contain numbers 1-9. Additionally, each row/column/region must contain all 9 colors, and each digit-color pair appears exactly once in the grid — the Graeco-Latin orthogonality constraint.
Is Suirodoku free to play?
Yes. Suirodoku is completely free to play online in your browser — no download, no registration required. Also playable as Tsuidoku on tsuidoku.com.
What are the exclusive techniques unlocked by the 4th constraint?
The 4th constraint unlocks Rainbow and Chromatic Circle techniques. Rainbow tracks a digit across all 9 colors to find the missing one — the digit reveals the color. Chromatic Circle does the reverse: track a color across all 9 digits to find the missing one — the color reveals the digit. Both techniques are exclusive to this color sudoku.
Is Suirodoku also called Tsuidoku?
Yes. Tsuidoku is the companion name for Suirodoku — 対 (tsui) means "pair" in Japanese, a direct reference to the 81 unique digit-color pairs at the heart of the game. Both names refer to the same free online color sudoku.