In an $n\times n$ board ($n\geq 3$), how many colors do we need so that we can color the cells such that no three consecutive cells (horizontal, vertical, or diagonal) are of the same color?
With three colors we can do it, using the pattern
$$131$$ $$232$$ $$312$$
and repeating it as necessary. Is it possible with two colors?
Yes you can:
Take $2$ rows of the form $1; 2; 1; 2 ;1; 2\dots$ and then $2$ rows of the form $2;1;2;1;2;1\dots$.
Repeat this until you've filled all rows