Simple coloring
Follows a chain of “one of these two” links for one digit.
A technique for Expert puzzles. Play a Expert puzzle
What it does
Pick a digit. Where a row, column or box has only two places for it, exactly one of them holds the digit. Link such pairs into a chain and colour it in two colours, taking turns. Either every cell of one colour holds the digit, or every cell of the other does.
How to spot it
Choose a digit with many “only two places” units. Start at one such cell, give it a colour, and follow the links, switching colours each time. Then look at what the colours see.
What it lets you do
A cell that sees both colours can’t hold the digit, because one of the colours is right. And if two cells of the same colour see each other, that colour is wrong everywhere, so the digit goes in every cell of the other colour.
Try it
Find the Simple coloring on this board. Pick the cells it clears, mark the candidates it removes, then check.
These are the notes after the easier steps: some candidates are already removed, as they would be in your game.
Practise it
The best way to learn a technique is to meet it in a real game. Practice puzzles are sure to need it, at Expert level; any Expert puzzle may need it too.