  For a positive integer $r$, let $G(r)$ be the smallest $N$ such that,
whenever the edges of the Cartesian product $K_N \times K_N$ are $r$-coloured,
then there is a rectangle in which both pairs of opposite edges receive the
same colour. In this paper, we improve the upper bounds on $G(r)$ by proving
$G(r) \leq \Big(1 - \frac{1}{128}r^{-2}\Big) r^{\binom{r+1}{2}}$, for $r$ large
enough. Unlike the previous improvements, which were based on bounds for the
size of set systems with restricted intersection sizes, our proof is a form of
a quasirandomness argument.
