It also fails because this highlighted triangle has all its corners yellow.

Your students first task is to try to solve this even larger triangle by coloring each vertex one of three colors. Try not go forward in this presentation till you give it a try.

Here is one solution.

Here is another solution. I like this one better because it has a kind of rotational symmetry.

Students who complete that first task can get the much more difficult task of solving this larger triangle. Again – try this before proceeding.

Here is an example of me failing. The next slide has my solution.

This took me a long time to find, but I’m sure it is not unique. There may be hundreds or thousands of answers.

The inspiration for this puzzle came directly from Bill Garach. In 2009 he started to ask questions like: Is it possible to color this 17×17 square with four colors so that every little rectangle doesn’t have all four corners the same color.

As you can see any rectangle oriented up-down does not have all four vertices the same color.

There are other interesting questions here. What happens if you are allowed to rotate the rectangle? What happens if the rectangle must be a square?

Bill Gasarch is actually interested in puzzles as “simple” as this attempt at 2-coloring this 5×5 square. Can it be done so that no rectangle has all four vertices the same. This example fails because all four corners are the same.