GridWits
Extreme tier Technique 20 of 46 Used by Smart Hint

Simple Coloring

Chain every cell where a digit has just two possible spots in its unit, alternating two colors — a contradiction or a shared sight line lets you eliminate.

How it works

  1. 1Digit 5 has exactly two homes in column A (A1 and A5), exactly two in row 5 (A5 and D5), and exactly two in the middle box (D5 and F4). Three strong links, chained end to end.
  2. 2Colour them alternately as the chain runs: A1 and D5 teal, A5 and F4 red. Exactly one of the two colours is the true one.
  3. 3F1 is outside the chain but sees a teal cell — A1, along row 1 — and a red one — F4, down column F. Whichever colour is true, F1 is staring at a 5, so it can't be one itself.

Play a Sudoku that needs this technique →

A 1 B 2 C 3 D 4 E 5 F 6 G 7 H 8 I 9 2 5 5 7 3 5 5 8 5 9 5 5 5 5 5
  • First colour in the chain
  • Second colour in the chain
  • Where the conclusion lands
  • Candidate ruled out
  • Strong link — At least one endpoint must be true.
Next in the ladder XY-Chain →

A longer alternating chain of links that traces a digit (or a sequence of digit pairs) across the board to find a single elimination.