Simple Colouring
Follow one digit's strong links across the board with two alternating colours, then look for contradictions.
What it is
Simple colouring tracks a single digit through units where it has exactly two candidate cells. Give one cell colour A and its partner colour B, then continue the chain: every strong link alternates the colour. Since one colour must be entirely true and the other entirely false, two conclusions follow. If two cells of the same colour share a unit, that colour is impossible. And any cell seeing both colours cannot hold the digit at all.
How to spot it
Pick a digit with many bi-location units. Start anywhere and colour alternately along the strong links. Then check for two same-coloured cells in one unit, and for uncoloured cells that see both colours.
Worked example
Step through the reasoning below. Each step adds one layer: first the position, then the pattern, then the candidates involved, and finally what can be eliminated.
Step 1 of 4: The starting position
- Pattern cells
- 1/7, 8/9, 1/9, 8/7
- Digits
- 5
- Eliminations
- 7/7
Press a marked square to point it out on the board above.
Why it matters
Colouring is the bridge from pattern recognition to chain-based solving, and it subsumes several named patterns: X-Wings, Skyscrapers and Kites are all special cases. Learning it once replaces memorising many shapes.
Frequently asked questions
What is a strong link exactly?
A unit where the digit has exactly two candidate cells. Exactly one of them is true, so the two cells always carry opposite colours.
What are the two possible conclusions?
First, if two cells of the same colour appear in one unit, that colour is false and every cell of the other colour is true. Second, any cell seeing both colours cannot hold the digit.
Is colouring the same as chaining?
It is the simplest form of it, restricted to one digit. Full chaining techniques follow links across several digits and are considerably more involved.
Put it to use
Where you will meet it
Expert grids are the only place this pattern regularly decides the solve. Bring your notes.
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