Paint the digit's either-or chain in two colours → erase the side that contradicts itself.
Technique 12 in the order the hint searches · difficulty Expert · up to advanced techniques
If a 3 has exactly two homes in some unit, those two cells are bound together as “if one is right the other is wrong”. Follow links like that, alternating colours, and a two-coloured chain grows across the grid.
Follow the places where a digit has exactly two homes
Fix on one digit and find units — rows, columns or boxes — where it has exactly two possible cells. Join those two and paint them differently. Keep going and the chain crosses the grid; around four cells in, it starts being worth something.
How the hint says it
The places where 3 has just two homes chain up in two alternating colours, and the same colour lands twice in column 4, at r4c4 and r8c4. One column cannot hold two 3s, so that colour is wrong outright and 3 comes off every cell of it.
One colour is entirely right and the other entirely wrong
Cells joined by the chain share a fate if they share a colour. That gives two conclusions: if the same colour appears twice in one unit, that colour is wrong outright; and any cell that sees both colours cannot be that digit whichever side turns out to be right.
The first technique that uses a chain. Where the earlier ones finish inside a single unit, this one runs clear across the grid.
Puzzles that need this technique
Four puzzles with a spot where nothing moves until you use Simple Colouring. Press a number to play it.
- Practice 1 ·
4-6586742 - Practice 2 ·
4-3201308 - Practice 3 ·
4-2072830 - Practice 4 ·
4-7687396