This method makes it possible to visualize all cells with a given value and to associate them.
A zone can be a row, column or block. For a selected zone, the complementary zone will be of different type.

If a zone has only two cells with a certain candidate, these cells are called combined or strongly linked. This is because we know that if the candidate appears or does not appear in any of the cells, we know automatically also for the other. "Colouring" is based on cells strongly linked. We must search chained cells closely linked, ie two chain cells where either all the first chain cells contain one the number and none of the other chain cell contain it, and reverse. All cells that belong to the 2 chains are the only solution, and cells that belong to none chain can not contain this number. The method is called "colouring" because if we colorized different chains, we see quickly chains strongly links and cells that are common.

This simple method can be used in the case of a difficult problem. It is selectable as Sudoku1 automatic solve methods in the Method panel by a click on Single.

You can also try to solve the Grid yourself, by displaying color chain with the command Color Chain (examples are proposed).

Example 1: 2 color chains in a line with figure "3"
Example 2: 5 color chains in a block for figure "4"
We can find 1 values of "4".