Sudoku

Simple colouring practice

Simple colouring follows one number through the units where it has only two places left. In each of those units, one cell holds the number and the other does not. Paint the cells in two colours, alternating as you follow the links.

One colour is true everywhere and the other is false everywhere. A cell outside the chain that sees both colours cannot hold the number. And if two cells of the same colour see each other, that whole colour is false.

Each line joins the only two places for a 5 in a row, column or box, so the colours alternate. One colour holds every 5. The red cell sees both colours, so it cannot be 5.

How to spot a colouring chain

Pick a number with several two-place units that connect to each other. Use the Colour button to paint the chain in two colours on the board, then look for a cell that sees one of each.

Practise it on this page

The board above deals positions where the Simple colouring is the only way forward, with every pencil note filled in. Cross off the candidates it rules out and press Next. Press Hint to see the pattern marked on the board.

In a full game, Master is the first level that can ask for it.