Simple coloring
Alternate-color a chain of strong links on one digit.
Simple coloring takes the strong-link idea and globalizes it. In any unit where a digit has exactly two candidate cells, one of those two cells must contain the digit — the cells are tied. Chain those ties across the grid for a single digit, and alternate-color the cells as you go (color A, color B, A, B…).
Once the chain is colored, any cell outside the chain that sees both an A-colored cell and a B-colored cell cannot be the digit. Whichever color turns out to be "true," the cell sees a true cell of that digit and is excluded.
It's the move that teaches you to see the grid as a network — and it's where expert-tier sudoku really begins.
When the move applies
After X-Wings and Y-Wings, build strong-link maps for digits that still have several two-candidate units. Chains form in the late middle game and unlock most of what's left.
The procedure
- Pick a digit.
- Find every unit where the digit has exactly two candidates — those pairs are strong links.
- Connect the strong links into chains; color the cells alternately A and B.
- Eliminate the digit from any cell outside the chain that sees both an A-cell and a B-cell.