Hard

Simple colouring

What it is

Follow a digit through units where it has only two places, colouring the cells in two alternating colours – one colour is the truth.

How to spot it

Start with a digit that has exactly two places in a unit and give them two colours. Continue through units where it has two places. A cell that sees both colours can't hold the digit.

Example

Basis
Second colour
Removed / placed

9 runs r1c1 – r5c1 – r5c6 – r2c6 through units where it has only two places. Either r1c1 and r5c6 (first colour) or r5c1 and r2c6 (second colour) are 9. r2c2 and r2c3 see one cell of each colour, so 9 is removed from them.

Used from level: Hard