by JasonLion » Sat Feb 07, 2015 1:32 pm
Coloring looks for two groups of cells where either one color of cells all contain the digit, or the other color of cells all contain the digit. the colored groups are built up from conjugate pairs, pairs where either one cell contains 4 or the other cell contains 4. Conjugate pairs are connected together when one cell of a pair can see a cell of a different pair, in which case those two cells must be different colors. To "see" a cell means the cells share a house.
In your picture R2C45 can both "see" a cell of each color (in this case R2C9 and R3C6). Since one color or the other must contain 4, no cell which can see both colors can contain 4.
In the picture, there are actually three colors shown. The two colors used for coloring are the brighter green and the blue/cyan. They have also marked several cells in a paler green to show all of the remaining cells containing a pencil mark for 4 that are not yet colored.
Last edited by
JasonLion on Sat Feb 07, 2015 4:35 pm, edited 1 time in total.