Sudoku Tutorial
Colors Technique
The COLORS technique is also called CONJUGATE PAIR COLORING.  It involves following a single candidate (6 in this example) and creating a chain of strongly linked pairs of cells.  Each pair is colored oppositely.   Once the chain  is compete, there are only two possibilities:  either all of the blue cells are 6's or all of green cells are 6's.    

If any cell in  the grid can 'see' two cells of opposite colors, that cell cannot contain the candidate in question.   In this example, the cells r8c5 and r9c5 (yellow) can 'see' the green cell in r1c5 and the blue cell in r4c5.  The 6's in the yellow cells can be eliminated.

        (In the Jet Sudoku program,  you can change cell color by pressing the Z key)
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