Simple coloring

Simple coloring builds a chain of 'strong links' on a single digit. In any unit where a digit has exactly two candidate cells, one of those cells must be the digit — so the two cells are tied in a strong link. Connect these links into chains and alternate-color the cells: color A and color B.

Now: any cell outside the chain that sees both a color-A cell and a color-B cell cannot be the digit. (Because one color is true and one false, whichever way it falls.) This is the most pedagogically rich elimination in advanced sudoku — it teaches you to see the whole grid as a network.

When to look for it

Once you're comfortable with X-Wings and Y-Wings, start mapping strong links for digits that appear in many partial units. Chains tend to emerge in the late middle game.

How to apply it

1. Pick a digit.
2. Find every unit where it has exactly two candidates — those pairs are strong links.
3. Build chains; alternate-color the cells (A and B) along each chain.
4. Find any cell outside the chain that sees both an A and a B — remove the digit from it.

Example