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.
When to use it
On expert and master puzzles when XY-Wing and Swordfish don't yield a move. Maintain accurate candidate maps and scan each digit for its strong-link network.
Worked example
Digit 5's strong-link network: R1C2↔R5C2 (column 2 strong link), R5C2↔R5C7 (row 5 strong link), R5C7↔R3C7 (column 7 strong link). Color R1C2=blue, R5C2=green, R5C7=blue, R3C7=green. Any uncolored cell that sees both a blue and a green cell drops 5 from its candidates.
Try it
The Sudoku247Online solver walks you through every move of any puzzle one logical step at a time, naming the technique that justifies each placement. Paste a puzzle to see this technique applied in real time.