Simple Coloring
Expert Removes candidates Used in: Sudoku, Killer Sudoku
Trace a digit through its conjugate pairs, alternately coloring the two ends of each strong link. If two cells of the same color see each other, that color is impossible; if a cell sees both colors, the digit is removed from it.
How to spot it
Follow a digit through its conjugate pairs, coloring the two ends of each strong link alternately.
What you can conclude
A cell seeing both colors can’t hold the digit; if two same-color cells see each other, that color is false.
Worked example
Color 4’s strong-link chain blue and orange; a cell that sees both a blue and an orange 4 cannot be 4.