Sudoku Coloring Technique: How to Eliminate Candidates With Simple Coloring

If you keep hearing about Sudoku coloring technique and it sounds more complicated than it needs to be, start here: coloring is just a way to track two possible states for the same digit. Instead of staring at long chains in your notes, you mark linked candidates with alternating colors and use the pattern to spot contradictions and eliminations.

This makes simple coloring Sudoku a useful bridge between intermediate solving and heavier chain-based logic. You do not need to memorize advanced theory first. You just need clean candidates, a single digit to track, and the patience to follow alternating strong links without guessing.

Sudoku Coloring Technique: Quick Answer

The Sudoku coloring technique marks alternating candidates of one digit across a chain of strong links. If two candidates of the same color see each other, that color must be false. If a cell sees both colors, that candidate can be eliminated from the cell.

Featured snippet answer: Sudoku coloring is a candidate-elimination method that uses alternating colors on linked candidates for one digit. The colors help solvers find contradictions or cells that see both color states, allowing safe eliminations without guessing.

What Is the Sudoku Coloring Technique?

The Sudoku coloring technique is an advanced note-based method. You choose one digit, find a chain of strong links, and color the linked candidates in alternating groups.

A strong link means the digit can appear in only two places inside a row, column, or box. If one of those two candidates is false, the other must be true. That is the foundation of coloring.

When you connect several strong links together, the candidates fall into two alternating sets:

• Color A
• Color B

Only one color set can represent the true placements for that chain. The point of coloring is not to decide which color is right immediately. The point is to use the two-color structure to prove that some candidates elsewhere must be wrong.

Why Simple Coloring Works

Simple coloring works because each strong link forces an either-or relationship. If one end of a strong link is false, the other end is true. When you keep alternating through a chain, every candidate in one color group shares the same truth state, and every candidate in the other group shares the opposite truth state.

That creates two main ways to make progress:

• Color trap: if an uncolored candidate can see one candidate of each color, it cannot be true, because one of those colors must survive.
• Color wrap: if two candidates of the same color see each other, that whole color must be false, so the opposite color becomes true.

This is why candidate coloring Sudoku can produce clean eliminations without trial and error.

How to Do Simple Coloring in Sudoku

1. Pick one digit

Choose a single number, such as 7. Coloring becomes messy if you jump between digits.

2. Find strong links

Look for rows, columns, or boxes where that digit appears in exactly two candidate cells. Those are the links you can trust.

3. Alternate two colors

Start with one candidate and mark it with Color A. Mark the other end of the same strong link with Color B. Continue across the chain, alternating each time the link forces the opposite state.

4. Check for a color trap

Now scan the rest of the grid for an uncolored candidate of the same digit that can see both colors. Because one color must be true, that outside candidate cannot also be true and can be removed.

5. Check for a color wrap

If two candidates of the same color end up seeing each other in the same row, column, or box, that color cannot be valid. The opposite color survives.

6. Rebuild the grid after eliminations

Coloring often creates only one or two eliminations, but those eliminations frequently unlock singles, locked candidates, or pairs on the next pass.

Simple Coloring Sudoku Example in Plain English

Suppose you are tracking candidate 4.

• In one box, the digit 4 can go in only two cells.
• One of those cells is also strongly linked to another 4 in its row.
• That second cell is strongly linked again through its box or column.

After a few steps, you have a chain of alternating 4 candidates colored blue and green.

Now imagine an uncolored cell in another part of the grid also contains candidate 4, and that cell can see one blue 4 and one green 4. That candidate must be wrong. If the blue path is true, the cell is blocked. If the green path is true, the cell is also blocked. So the outside 4 can be eliminated.

You still are not guessing which color wins. You are only using the fact that one of them must win.

Color Trap vs Color Wrap

Color trap

A color trap happens when an uncolored candidate sees both colors. That outside candidate gets eliminated.

Color wrap

A color wrap happens when two same-colored candidates see each other. That color contradicts Sudoku rules, so every candidate in that color set becomes false.

For most improving solvers, the color trap is easier to spot and safer to learn first. It is the version that makes simple coloring Sudoku feel practical instead of theoretical.

When Should You Use the Sudoku Coloring Technique?

Use coloring after simpler methods stop producing progress. It is most useful when:

• your candidate notes are already clean,
• you have exhausted singles, pairs, and locked candidates,
• the same digit keeps forming strong links across several houses, and
• you want logic stronger than basic pattern spotting but lighter than long forcing chains.

If you are not yet comfortable with notes, start with How to Use Notes in Sudoku before relying on coloring.

Sudoku Coloring Technique vs Chains

Coloring and chains are closely related. Both follow logical links between candidates. The difference is mostly in presentation.

• Coloring makes the two possible truth states visual.
• Chains are often written as explicit step-by-step logic paths.

That is why many solvers learn coloring before deeper chain techniques. It turns the same logic into something easier to scan.

Common Mistakes With Candidate Coloring Sudoku

• Using weak links as if they were strong links: if a row or column has three or more possible places for the digit, the link is not forced.
• Coloring more than one digit at once: that usually creates confusion instead of clarity.
• Forgetting box links: strong links can come from boxes too, not just rows and columns.
• Eliminating from cells that do not see both colors: the outside candidate must see one candidate from each color state.
• Starting too early: if easier logic is still available, coloring is often unnecessary.

A Practical Checklist

1. Choose one digit.
2. Mark only strong links for that digit.
3. Alternate two colors across the chain.
4. Look for a color trap first.
5. Then check for a color wrap.
6. Apply eliminations and rescan the grid for simpler follow-up moves.

For related logic, see What Is a Bivalue Cell in Sudoku? and Sudoku Patterns to Look For.

FAQ: Sudoku Coloring Technique

What is the Sudoku coloring technique?

The Sudoku coloring technique is a way to mark alternating candidates for one digit across strong links. It helps you find contradictions and eliminate candidates without guessing.

Is simple coloring hard to learn?

It is easier than it sounds if your notes are clean. Most players struggle more with messy candidates than with the actual coloring logic.

Do I need pencil marks for Sudoku coloring?

Yes in most cases. Coloring depends on exact candidate positions, so it is difficult to use without notes.

What is a color trap in Sudoku?

A color trap happens when an uncolored candidate sees one candidate from each color group. Because one color must be true, the outside candidate can be eliminated.

What is a color wrap in Sudoku?

A color wrap happens when two candidates of the same color see each other. That proves the entire color is false, so the opposite color must be true.

Conclusion

Sudoku coloring technique is useful because it turns abstract chain logic into something visual and manageable. You are not trying to predict the whole puzzle. You are only using alternating truth states to remove bad candidates safely.

Start with one digit, follow only strong links, and look for color traps before anything else. Once the first elimination lands, the rest of the puzzle often becomes much easier to read.

If you want to practice it on harder boards, try a tougher grid at Pure Sudoku and compare the result with the site’s other advanced solving guides.