Simple Colouring paints the strong-link graph of one digit with two colours — say blue and green.
On a strong link, the two ends have opposite colours: exactly one is true. This is a gateway to Medusa and AIC thinking.
Why it works
If two cells of the same colour see each other, that colour cannot be true (two truths of the same digit would see each other). So the whole colour is false — the opposite colour is true.
If a cell sees both colours, it sees a true candidate either way — so it cannot be the digit.
Colouring turns a web of “exactly one of these” links into simple paint rules.
How to spot it
Build the strong-link graph from bi-locals first. Same-colour conflict or see-both-colours are the traps to watch for.
Demonstration
For one digit, connect every strong link (exactly two candidates in a house) into a chain or network.
Colour the network with two colours. Opposite colours mean “exactly one is true.” The green cells show a useful colouring on 7.
Watch for same-colour conflicts or cells that see both colours — those are the traps.
Eliminate 7 where colouring forbids it (red).
Example 1
Colour the strong-link network for one digit.
Colouring on 1 finds a contradiction or trap.
Remove 1 where the paint rules say no (red). Same-colour conflict or see-both means elimination.
Example 2
Colour the strong-link network for one digit.
Colouring on 2 finds a contradiction or trap.
Remove 2 where the paint rules say no (red). Same-colour conflict or see-both means elimination.