Colouring is a technique that can become quit hard. This time you will be following numbers that is two in a row/column/Block. Two colours per chain.
Colours change as you go down the chain. In the example below three chains were found. The Red Yellow found the elimination.
For the number 4 a chain was found that resulted in Row(10) Column(6) The starting point and Row(5) Column(8) to be of opposing colour.
Row(1) Column(8) Therefore can not be a 4 as one in the chain of the two will be 4.