  There are two conjectures concerning planar graph colourings that are
strengthenings of the four colour theorem. One concerns signed graph colouring
and is proposed by M\'{a}\v{c}ajov\'{a}, Raspaud and \v{S}koviera. It asserts
that every signed planar graph is $4$-colourable. Another concerns list
colouring and is proposed by K\"{u}ndgen and Ramamurthi which asserts that if
$L$ is a $2$-list assignment of a planar graph $G$, then there is an
$L$-colouring of $G$ such that each colour class induces a bipartite graph. In
this note we prove that the first conjecture implies the second one.
