Here you will find that the chain will have two cell with the same colour in the same row/column/Block. That state is thus not possible.
All numbers of that specific colour can be eliminated. Marked with BROWN
See RED YELLOW chain with Yellow On Yellow in Row(3)
Look : It also have a colouring (1) in Row(1) Column(6)
There are four chains that the program will follow 1 2 : 3 4 : 5 6 : 7 8