  A signed graph is a simple graph with two types of edges. Switching a vertex
$v$ of a signed graph corresponds to changing the type of each edge incident to
$v$.
  A homomorphism from a signed graph $G$ to another signed graph $H$ is a
mapping $\varphi: V(G) \rightarrow V(H)$ such that, after switching any number
of the vertices of $G$, $\varphi$ maps every edge of $G$ to an edge of the same
type in $H$. The chromatic number $\chi_s(G)$ of a signed graph $G$ is the
order of a smallest signed graph $H$ such that there is a homomorphism from $G$
to $H$.
  We show that the chromatic number of signed triangular grids is at most 10
and the chromatic number of signed hexagonal grids is at most 4.
