Simple Coloring
Simple Colors · Single's Chains
Join a digit's strong links into a chain and colour it in two alternating colours: exactly one colour is the answer.
Rule
A cell that sees both colours → remove d. Two cells of the same colour that see each other → that colour is false.
Join the strong links
For digit 2, find the units with exactly two places left for 2 and connect those two places. The links join up into a chain of 8 cells.
The idea
Pick a digit d. Every unit with only two places for d gives a strong link: exactly one of the two cells is d. The links join up into chains.
Colour along the chain alternately: blue, orange, blue, orange… Since each link has exactly one end that is d, either all blue cells are d or all orange cells are d.
There are two rules. Trap: a cell outside the chain that sees both a blue and an orange cell sees a d whichever colour is true — eliminate. Wrap: two cells of the same colour that see each other mean that colour cannot be true — remove d from all cells of that colour, and every cell of the other colour is d.
The Skyscraper and the 2-String Kite are really short colouring chains.
How to spot it
- Pick a digit with many strong links, and colour it with two pens right on the paper.
Common mistakes
- Join cells only with strong links. Joining two cells that merely see each other (a weak link) leads to wrong conclusions.
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Related techniques
References
- HoDoKuSimple Colors (Color Trap, Color Wrap)(opens in a new tab)
- SudokuWikiSimple Colouring (Rule 2, Rule 4)(opens in a new tab)
The examples on this page are generated from random puzzles, not taken from these sources.