Tricky problem on pigeonhole principle

Question

The question was asked in my today's quiz and I have no idea how to start with it. It's evident that we have to use pigeonhole principle somehow but how I am not getting. Question is " A 4×9 rectangular board is divided into squares each of which is coloured red or green or blue. Prove that the board contains a rectangle whose four corner squares are all the same colour. Thanks.

Answer 1

Counter Example ... for instance ...