Irodoku — The Color Sudoku Without Digits

In Irodoku, nine colors take the place of the digits 1–9. Each row, column, and 3×3 box must contain all nine colors exactly once. The constraint structure is identical to standard Sudoku: 27 constraints, nine distinct symbols.

What changes is the experience. Without digits, you lose sequential reasoning ("1 comes before 2") and rely on pattern recognition instead. The puzzle isn't harder — it's a different cognitive mode.

➡️ Quick Recap: Irodoku = Sudoku with colors instead of numbers. Same rules, same structure, different cognitive experience.

Suirodoku — The 4th Constraint

Every cell contains both a digit (1–9) and a color (one of nine). Digits follow standard Sudoku rules. Colors follow the same rules independently. A global rule ties them together: each of the 81 digit-color pairs appears exactly once.

This is a Graeco-Latin square, a mathematical structure first studied by Euler in 1782.

Rule Irodoku Suirodoku
Digits in cellsNoYes (1–9)
Colors in cellsYes (replace digits)Yes (alongside digits)
Digit constraints (row/col/box)—Yes
Color constraints (row/col/box)YesYes
Global pair uniqueness—Yes (81 pairs)
➡️ Quick Recap: Suirodoku = digits + colors as parallel constraints + global uniqueness. Mathematically a Graeco-Latin square.

How Solving Differs

In Irodoku, you solve one constraint set with visual symbols. In Suirodoku, you solve two constraint sets that interact. Placing a digit can reveal a color, and vice versa.

This enables two techniques with no equivalent in Irodoku or classic Sudoku:

  • Rainbow Technique — track one digit across all 9 colors to locate the missing pair.
  • Chromatic Circle — track one color across all 9 digits to eliminate possibilities.

Both rely on global pair uniqueness. They can't work in Irodoku because there are no pairs — just single symbols. A structured training plan covers these techniques from easy to expert.

💡 Key Point

More constraints means more information per cell — which means more deductions at each step, not just more difficulty.

➡️ Quick Recap: Irodoku = one constraint set. Suirodoku = two interacting sets, enabling Rainbow and Chromatic Circle techniques.

The Sudoku with a 4th Constraint

Exclusive solving techniques
Strategies you won't find elsewhere
Compete worldwide
Secured scoring & global leaderboard
Play anywhere
Mobile or desktop — progress syncs
100% free
No ads, no paywall
▶ Play Now

Frequently Asked Questions

Is color sudoku harder than regular Sudoku?
Irodoku shifts difficulty from numerical to visual — different, not necessarily harder. Suirodoku has more constraints but also more information per cell. The difficulty depends on the grid, not the format.
What's the difference between Irodoku and Suirodoku?
Irodoku replaces digits with colors — same Sudoku structure. Suirodoku keeps digits and adds color as a 4th constraint with global pair uniqueness — different techniques, different game.
Do I need to know Sudoku first?
For Irodoku, basic familiarity helps. For Suirodoku, you should be comfortable with standard Sudoku solving before adding the color layer.