Six points connected in pairs by coloured lines Six points are connected in pairs by lines each of which is either red or blue. Every pair of points is joined. Determine whether there must be a closed path having four sides all of the same colour. A path is closed if it begins and ends at the same point.

Another proof I found on http://staff.imsa.edu/~nprince/teachingfiles/S10/GTwA/:
Let the edges of $K_6$ be
$2$-colored, and let $v\in V(K_6)$. Suppose that neither color
contains a monochromatic $C_4$. Notice that if $u$ and $v$ are
vertices such that $u$ sends edges of color $1$ to every vertex of
$W=\{w_1,w_2,w_3\}$ and $v$ sends edges of color $2$ to every vertex
of $W$, then regardless of how the edges among the $w_i$'s are
colored, there will be a monochromatic $C_4$. If $v$ is incident to
at least $4$ edges of color $i$, then there must be a $u$ and a set
$W$ such that $u$ sends three edges of color $3-i$ and $v$ sends
three edges of color $i$ to $W$, and we are done by the previous
sentence. Therefore, we can assume (by symmetry) that $v$ is
incident to exactly $3$ edges of color $1$. Call the set of
neighbors of $v$ in color $1$ $W$. Let $\{x,y\}=V(G)-W-v$. If $x$
and $y$ have a common neighbor in color $2$ in the set $W$, then
there is a monochromatic $C_4$ in color $2$. Otherwise, one of $x$
and $y$ has sends two edges of color $1$ into $W$, yielding a $C_4$
in color $1$.
