sudokUI › Sudoku techniques › Simple Colors
Simple Colors in sudoku
all X-Chains in one cluster · how it fits
For one digit X, cells joined by conjugate pairs form a cluster, coloured in two colours so that the two cells of every conjugate pair get opposite colours.
Why it works
One colour holds X in all its cells, the other in none. If two cells of one colour see each other, that colour is false and X is removed from all its cells; any uncoloured cell that sees cells of both colours also loses X.
How to spot it
Pick a digit, mark every unit where it has exactly two places, and follow pairs that share a cell to build the cluster.
Worked example
Simple Colors on 4: its conjugate pairs (the solid links) are coloured blue and gold, and either every blue 4 is true or every gold one is. The 4 in r6c8 sees a blue 4 in r6c4 and a gold 4 in r8c8, so it is false either way.
Practise it on a real puzzle. sudokUI generates a puzzle that needs Simple Colors and takes you straight to the position where it applies. Hints draw the pattern on the board and explain the step.
In a puzzle's rating
Each time the solver needs Simple Colors, the puzzle's difficulty rating grows by 150. Its class is Hard. It is needed in 1 in 520 puzzles that sudokUI generates. How the rating works.