How it works
Coloring tracks a single digit through chains of bivalue positions. Two cells in the same unit that are the only places a digit can go are called strongly linked. They have opposite truth values: exactly one of them holds the digit.
Color one cell white, its partner black. Find other cells strongly linked to those, and alternate. If at any point two cells of the same colour share a unit, that colour cannot hold the digit anywhere in the chain. Every cell of that colour can be eliminated.
If a cell outside the chain sees both colours, it cannot hold the digit regardless of which colour is correct. Eliminate it.
When to look for it
Late in expert and evil puzzles when one digit dominates the remaining candidates and forms long chains.
Tips for spotting the pattern
- Pick a digit with many bivalue positions left. Long chains are most productive.
- Coloring is most useful after pairs and pointing pairs have been exhausted.
- Multi-color extensions (three or more colours) handle more complex chains but the logic is the same.
Common mistakes
- Coloring across weak links. The two cells must be the only candidates for that digit in a shared unit.
- Eliminating a cell that only sees one colour. It must see both.
- Missing the contradiction. If two same-colour cells share a unit, every cell of that colour can be erased.