This actually belongs to "pigeonhole principle"

There are four red and yellow combinations * * * and nine columns * * *.

This is equivalent to four drawers and nine things. If you put nine things in a drawer, there will be at least three things in at least 1 drawer.

The understanding of its principle is also very simple. If arranged according to the least overlapping situation, each painting method is used once on average, which is four columns. If you use it twice, it is eight columns. No matter how you draw it, the remaining ninth column will be repeated with one of the four.