The 4th rule of color sudoku is simple: each color must contain numbers 1-9. Combined with the 3 classic sudoku rules, this creates 81 unique number-color pairs and changes how you search for solutions.
Sudoku Has 3 Rules
Sudoku Has 3 Rules
Every Sudoku player knows the classic constraints:
- 1 Each row must contain numbers 1-9
- 2 Each column must contain numbers 1-9
- 3 Each 3×3 region must contain numbers 1-9
These three rules have defined Sudoku for decades. But what if there was more?
In one sentence: The 4th rule states that each of the 9 colors must contain all numbers from 1 to 9 — just like rows, columns, and boxes.
Suirodoku Adds a 4th
Suirodoku Adds a 4th
4
The game-changing rule
- 1 Each row: numbers 1-9 + all 9 colors
- 2 Each column: numbers 1-9 + all 9 colors
- 3 Each 3×3 region: numbers 1-9 + all 9 colors
- 4 Each color must contain numbers 1-9
The 4th rule means: All 9 Green cells together must contain 1, 2, 3, 4, 5, 6, 7, 8, 9. Same for Red, Blue, Yellow, and every other color.
Why It Matters
Why It Matters
This extra constraint creates a fascinating consequence:
- 9 colors × 9 numbers = 81 unique pairs
- Each number-color combination appears exactly once
- There is only one "Green 5", one "Red 3", one "Blue 7"...
This uniqueness enables solving techniques that are impossible in regular Sudoku.
New Solving Power
New Solving Power
With the 4th rule, you can:
- Track a number across colors: If you find eight 4s in eight different colors, you know exactly which color the 9th must be (Rainbow Technique)
- Track a color across numbers: If Green has 1-8, the missing cell must be Green 9 (Chromatic Circle)
The 4th combination doesn't make Suirodoku harder — it gives you more tools to solve it.