Simple Coloring
Color the strong-link chain for one digit alternately blue and green. Any cell that sees both colors loses that digit.
Advanced sudoku technique
What it is
Simple Coloring (also called Singles Chain) is the simplest parity-based chain technique. Pick a digit. Find every 'strong link' on that digit — a unit (row, column, or box) where the digit has exactly two candidate cells. Color one end of each strong link blue and the other green. Walk the network: every cell sharing a strong link with a colored cell takes the opposite color. After coloring, one of the two colors MUST hold the digit and the other CANNOT. We don't know which is which yet, but two consequences follow: 1. Any uncolored cell that sees both a blue cell AND a green cell loses the digit — one of the two colors holds it, and it would see that placement either way. 2. If two cells of the SAME color sit in the same unit, that's a contradiction — so that color is the empty set, and every cell of the OTHER color holds the digit.
Want the full theory?
The Simple Coloring guide on Sudoku247Wiki walks through the logic step by step, with worked examples, diagrams and FAQs.
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